MOLLUSCA. 203 



several individuals ; each having its own heart, respiration, 

 and system of nutrition, but fixed on a peduncle that branches 

 from a common creeping stem, and all being connected by 

 a circulation that extends throughout. Their parts of such 

 transparency that their interior is easily seen. Their ex- 

 ternal shape is that of a pouch, compressed at the sides, and 

 fixed at the hind part of its base upon the peduncle. 



Its two openings are in the form of very short tubes ; that 

 of the mouth at the top of the pouch, and that of the funnel 

 in front. The longest diameter, from the peduncle to the 

 space between the openings, is about -085 inch. 



The outer covering is a tough coat, a continuation of the 

 peduncle, more pliable near the openings ; lined interiorly 

 with a soft substance or mantle, in which a ramifying circu- 

 lation is very distinct. A great part of the interior is occu- 

 pied by the branchial sac, which is subcylindrical, flattened 

 at the sides, and has its axis vertical ; its cavity terminating 

 upwards in the oval opening, and being closed at the bot- 

 tom. It is united to the envelope, or to the mantle above 

 and behind ; the juncture, beginning in front of the oval 

 opening, extends backwards on each side of it, and then 

 downwards in two lines ; between these, along the middle 

 of the back, is a vertical compound stripe, that seemed to 

 me cartilaginous. At the bottom the sac appears to be en- 

 veloped by the soft substance of the mantle, but at its sides 

 and front a vacant space is left between them, that ends in 

 the opening of the funnel. The branchial sac is more com- 

 pressed towards its lower part ; and here are placed, exter- 

 nally to it, the heart on the left, and the stomach and other 

 viscera on the right side, the vent opening upwards at the 

 front into the funnel. On its sides and front the sac is per- 

 forated by four rows of narrow, vertical, irregularly oval 



