EELATION OF ARTHROPOD HEAD TO ANNELID PROSTOMIUM. 253 



uature of a blood-space, most clearly seen in trochosphere 

 larvae, where it is much enlarged. Although later in develop- 

 ment in both 01igocha3ta and Poiychjeta, the prostomial cavity 

 becomes confluent with the coelom of the peristomium. It is 

 only coelomic by virtue of this connection.^ No nephridia are 

 developed in the prostomium, and on its upper surface is 

 formed the primitive brain or supra-cesophageal ganglion. This 

 brain may develop from the first as a single median structure, 

 or may originate from several centres connected with the 

 organs of sense, which subsequently become fused (Kleinen- 

 berg, Racovitza). We see that not only does the prostomium 

 in Annelids differ from a metamere in size and shape, but it 

 never at any time during its development exhibits the cha- 

 racters of a true segment. It therefore cannot be considered 

 as a reduced or vestigial metamere. Can it be considered as 

 an incipient segment? Obviously not, except on the very 

 strongest evidence, since such a fact as the growth of a new 

 segment in front of the first metamere would be opposed to 

 the rule which is known to hold good for Annelid segmentation 

 (Milne-Edwards' law). Such evidence is entirely wanting. 



The only other opinion that can be held is that the pro- 

 splanchnic layer of the cephalic ring, which at first covers only the upper side 

 of the buccal fossa and oesophagus with a tliick layer of mesoderm, extends 

 gradually its lateral parts towards the central [ventral ?] surface, and embraces 

 the ingestive aperture completely." And again, "The anterior end of the 

 head segment becomes more and more prominent, and is transformed into a 

 cylindrical process, the upper lip, a kind of proboscis " = prostomium ? 



' As Vejdovsky says (p. 320, 16), " die Kopfhohle [peristomial cceiom] 

 sowohl von Rhynchelmis als der Lumbriciden weicht also genetisch nicht von 

 Leibeshohle der nachfolgenden Segmente als. Sie wachst erst nachtraglich 

 zum sog. Prsestomium aus, welches letztere daher nicht als Kopf, sondern 

 als ein Kopf-fortsatz oder Kopflappen aufzufassen ist. Dafiir sprechen 

 zuerst die embryologischen Thatsachen bei Lumbriciden, wo der Mund am 

 vorderen Korperende terminal nach aussen miindet, und erst nachtraglich 

 durch den sich verlangernden Kopflappen von der Riickenseite verdeckt wird. 

 Dies allerdings erst sehr spat, nachdem das Kopfganglion liingst angelegt 

 ist und sich somit nicht im Praestomium bilden kann, wie unlangst von einer 

 Seite behauptet wurde." With regard to the latter fact, the brain neverthe- 

 less arises from the prostomial region, although the prostomium may be 

 retarded in development. 



