210 COMPARATIVE ANATOMY chap. 



cloaca. The male genital aperture may even be combined with the mouth {e.g. in 

 Stylostonium among the Polyclada and in Prorhynchiis among the Rhahdoceila). 

 In the Mesostoinidce among the Rhabdoccela the longitudinal branches of the water- 

 vascular system, which generally emerge independently at the surface of the body, 

 enter the pharyngeal pouch, which itself represents an invagination of the outer 

 body wall [stomodoeum). In a few jSfemeriina the otherwise universally separated 

 prohoscidal and oral apertures may be united (Amphiporus, Malambdella, Geonemertes 

 palaensis). 



The following are the most important combinations of the anal 

 aperture with the apertures of other organs. There are three kinds 

 of such combinations. 



1. Union of the anal aperture with the apertures of the 

 nephridia or excretory organs. — In the PriapulidcB the two anal 

 organs which serve primarily for excretion and secondarily as genital 

 ducts emerge quite near the anus. In Sipimculus slightly developed 

 anal glands have been observed entering the end of the hind-gut ; 

 these are perhaps (?) homologous with the anal organs of the Priapu- 

 lidce. Li the Echiuridce, which in many ways occupy a position inter- 

 mediate between the Chcdopoda and the Sipunmlacm, both the anal 

 glands (anal nephridia) also enter the hind-gut. In the Rotatoria also 

 anus and nephridia enter a common cloaca. 



2. Union of the anal with genital apertures. — In the male 

 Xenadoda the genital aperture emerges ^Wth the anus into a common 

 cloaca ; and the same is the case in the female Botatoria. 



3. Union of the anal with both nephridial and genital aper- 

 tures. — This occurs, as may be seen from 1 and 2, in the female 

 Rotatoria, where all three apertures open into a common cloaca. 



The position of the anus and the cloaca. — The anus has a 

 terminal position in the Nemertina, Nematoda, and Annulata. In 

 the Nematoda it lies ventrally at a little distance from the posterior 

 end of the body ; in the Annulata generally dorsally, but always in 

 the last segment, except in Notopygos {Amphinomidce), where it lies 

 several segments from the posterior end on the back. In the 

 Chcetognaiha it lies ventrally at the boundary between the trunk and 

 the tail regions ; in the Rotatoria dorsally at the boundary between 

 trunk and foot. In attached tubicolous Rotatoria it has moved some- 

 what forwards on the back, so that the hind-gut bends round for- 

 wards. The Prosopygia are distinguished by the fact that the anus 

 lies on the back, moved far forward ; and in the Sipxmmlidm it even 

 lies at the boundary between proboscis and trunk ; in Phoronis and the 

 Bryozoa quite anteriorly on the back, either (Edoprocta) outside the 

 tentacle carrier or (Endoprocta) inside it. Where an anal aperture is 

 retained in the Brachiopoda it lies anteriorly, to the right, near the 

 mouth. There are, however, a few exceptions to the rule which 

 generally holds for the Prosopygia. In the Priapulidce and in the 

 BracJiiopod genus Crania the anus lies behind. 



