On the Ancestral Form of the Chordata.

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366 A. A. W. HUBRECHT. In Neraertines these diverticula become constricted off from their point of origin—the oesophagus—and entering into connection with invaginations from the epiblast, which bring about a free access of the external sea-water, they become converted—at least in the large section of the Schizonemertines —into an apparatus which I have proved to be subservient (' Zur Anatomie u. Physiologie der Nemertinen,' p. 28) to a process of cerebral respiration, in which oxygen is carried to the nervous system itself, the cellular elements of which are in this subdivision profusely provided with haemoglobin. I am not prepared to say that in the great subdivision of the Hoplonemertini, where the central nervous apparatus is no longer provided with haemoglobin, but where, on the contrary, the circulating fluid is, these diverticula, which continue to develop in the same way out of the oesophagus, are also—and in the first place—subservient to a respiratory process. I am rather inclined to believe that in this group the cephalic grooves—as the epiblastic invaginations travelling inwards to meet the bypoblastic diverticula in question are called—remain more especially adapted for sensiferous purposes, probably of olfactory nature. The way in which the complicated organs in the adult, the socalled side organs, develop, remains quite the same: an outgrowth from the oesophagus coalesces with an ingrowth from the epiblast, the principal difference being that the connection with the brain-lobes is no more so intimate, and that the apparatus is connected with the brain by a special set of nerves. In some species it continues to be situated behind the brain, in others it becomes placed in front of the central nervous apparatus. It appears to me that these facts are not without significance. However, I refrain for the present from a further discussion, and would merely wish to point to an interesting detail in the development of Amphioxus lately brought to light by Hatschek's researches. It is the presence in the anterior region of the oesophagus, in front of the mouth, of two lateral hypoblastic diverticula, differing in their nature and in their further development, both from the archenteric diverticula (mesoblastic somites), and from the branchial outgrowths of
ON THE ANOESTEAL FORM OF THE OHOBDATA. 367 the oesophagus. These two diverticula, originally symmetrical, become constricted off from the hypoblast, and in their further development they have a different fate, the left one communicating with the exterior by a ciliated opening, which appears in the epiblast, the right one forming an epithelial lining in the prseoral body-region. The left one was looked upon by Kowalevsky as a special sense-organ of the larva. Although I am not prepared for the present to furnish any evidence in this direction, I would call attention to the similarity in development between these structures and the cephalic diverticulum of Neniertines. Considering the amount of degeneration which in several respects Amphioxus appears to have undergone, it does not appear impossible that the left lobe is really a temporary olfactory organ, the right one having entered upon other functions, and having lost its original significance. These cssophageal diverticula of Amphioxus would stand about in the same relation to the posterior paired outgrowths of the cesophagus which ulteriorly give rise to the branchial slits of this animal, as would the two primary larval diverticula of Balanoglossus, giving rise to the first pair of gill-slits, to the following ones successively appearing behind them. In Nernertines only one corresponding pair of respiratory diverticula is encountered, and they may remain in connection with those portions of epiblastic ingrowths which form the primary constituents of a sensorial (olfactory?) apparatus in certain of the higher differentiated genera in the way we have above traced. The far reaching significance of our starting point has obliged us to throw a rapid glance at the principal points in which Nemertines allow of a certain degree of comparison with Vertebrates, and it would lead us too far off if we were to follow this up for the secondary, less important points, or for those which are at present not fully enough known to allow of any fruitful comparison. Among the latter I count the excretory and the generative apparatus. Do the closed generative sacs
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On the Ancestral Form of the Chordata.

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