164 THE ANATOMY OF IN VERTEBRA TED ANIMALS. 



consisting of a sharp cbitinous style (Fig. 34, J5). There is 

 no buccal proboscis, but the mouth leads into a long, straight 

 intestine, with short, lateral, caecal dilatations. 1 



The JProctucha usually present only the pseud-haemal ves- 

 sels, though, as has been mentioned above, Schulze found water- 

 vessels coexisting with them in Tetrastemma (Fig. 34). 



The nervous system of the Proctucha is like that of the 

 Aprocta y but, in correspondence with the often extreme elon- 

 gation of the body, the backwardty- prolonged cords are very- 

 stout. Moreover, the ganglia are united by an additional 

 commissure over the proboscis, which thus traverses a ner- 

 vous ring. In some, the lateral cords approach one another 

 on the ventral aspect of the body, and ganglionic enlarge- 

 ments appear where the nerves are given off, thus present- 

 ing an approximation to the double ganglionated chain of 

 higher forms. 



In addition to eyes, almost all the JProctucha possess two 

 ciliated fossae, one on each side of the head (Fig. 34, bb), 

 which receive nerves from the ganglia. Occasionally two 

 otolithic vesicles are attached to the cerebral ganglia. 



The Proctucha are almost always dioecious. The simple 

 reproductive glands are lodged in the intervals between the 

 saccular dilatations of the intestine, and the ova and sper- 

 matozoa usually make their way out by the dehiscence of 

 the integument. In some, however, the embrvos are devel- 

 oped in the ovarian sacs, or in the cavity of the body. In 

 most of the Proctucha, the egg, after passing through the 

 morula stage, acquires an alimentary cavity, apparently by 

 delamination, and passes, without other metamorphosis than 

 the shedding of a ciliated outer investment, into the form of 

 the adult. 



Prof. A. Agassiz 2 has described a free-swimming larva, 

 the broad anterior end of the body of which is surrounded 

 bv a zone of cilia, immediately behind which the mouth opens ; 

 while around the anal aperture, at the narrow posterior end, 

 is a second circlet of cilia. This larva exactly resembles 

 those forms of polychaetous Annelidan larvas which are called 

 Telotrocha. As in these Annelids, the region of the body 

 which lies between the two ciliated rings elongates and be- 

 comes segmented, while a pair of eyes and two short tenta- 



1 For the organization of the Rhynchocoele Turbellaria, or Nemerteans, see 

 Dr. C. Mcintosh's elaborate monograph lately published by the Ray Society. 



2 "On the Young Stages of a few Annelids." (Annals of the' Lyceum of 

 New York, 1864.) 



