MOLLUSCA. 515 



71. DIDEMXFAI. Individuals in indistinct systems. 



The thorax is short and subglobular. The abdomen in- 

 i-j pedunculated, and larger than the thorax. The 

 anal opening is obscure. The ovarium is single, sessile, and 

 placed on the side of the abdomen. I), candidum and vix- 

 cosum, from the Gulf of Suez, are the only known species. 



II. Branchial Orifice simple. 



The species appear in the form of thin fleshy crusts on 

 stones and sea-weeds. The individuals are stellularly ar- 

 ranged in distinct systems. The branchial orifice is cir- 

 cular, and undivided. The abdomen is sublatcral, and 

 fixed at the bottom of the branchial cavity. The intes- 

 tine is small, and the anus indistinct. The angles of the 

 branchial meshes are without papillae. 



73. BOTRYLLUS. Systems furnished with a central ca- 

 vity. 



The systems are prominent, and consist of one or more 

 regular concentric rows. The ovarium is double, being 

 attached to each side of the branchial sac. 



This genus is subdivided by SAVIGNY into Botryli steL 

 lati, and Botrylli conglomerati. In the first, where the 

 individuals are distributed in a single row, there are some 

 species in which the individuals are cylindrical, with ap- 

 proaching orifices, and the limb of the central cavity not 

 apparent after death and probably short, as the B. rosacevs 

 Leachii and Borlasii. In other species, the individuals are 

 ovoid, with remote orifices, and the limb of the central cavi- 

 ty is always apparent and notched, as B. Sclilosseri, stellatus, 

 gemmeus, and mmutus. In the Botrylli conglomerati, in 

 which the individuals are disposed in several rows, there is 

 only one species, B. conglomeratus. 



74. EUC^ELIUM. Systems destitute of a central cavity. 

 The individuals are distributed in a single row, and the 



ovarium is single, sessile, and attached to the side of the 



K k 2 



