Introduction to Animal Morphology. 241 



the common cloaca, the abdomen not sharply marked off; 

 they may be regularly arranged (Botryllus) in stars or discs, 

 or irregularly (Botrylloides). (B.) Didemninas — having the 

 body divided into thorax and abdomen, and each embryo 

 producing two individuals. In Leptoclinum there are few 

 systems and a six-lobed mouth. Encoelium has many systems 

 and an obsoletely rayed mouth. Didemnium has many 

 systems and a pedunculate abdomen. Distomus has one or 

 more circles of personse, and each has a six-rayed mouth and 

 anus. Diazona has regular concentric circles of personse 

 forming a rosette-like disc. (C.) Polyclininag — body with 

 not only a thorax and abdomen, but a post-abdomen, con- 

 taining the heart. Polyclinum has a pedunculated post-abdo- 

 men ; the mouth is six-rayed ; the anal opening prolonged 

 horizontally and irregularly cut. Aplidium has the anal 

 opening indistinct, and no central cavity. Sidnyum has an 

 eight-toothed branchial opening, and a simple, tubular, folded, 

 anal opening. Synoicum has a six-rayed mouth, an irregu- 

 larly six-rayed anus, and a sessile post-abdomen. Amouro- 

 cium has indistinct abdominal divisions; mouth six-rayed. 

 In Parascidium the mouth is eight-rayed, with two eye-specks. 

 Sigillina is seal-like, the narrowing circlets rising one over 

 another. 



CHAPTER XXXIV. 



SUB-KINGDOM 6. — MOLLUSCA [Cumcr]. 



Mostly aquatic, soft-bodied personae, composed of 

 one, two, or three obscurely marked metameres, with 

 a concealed bilateral symmetry. Their structure can 

 be easily understood by regarding them as Vermes* 



* We know as yet of no absolute passage forms or direct synthetic 

 tj'pes. 



R 



