LECTURES ON MOLLUSCA. 129 



lent and ex cur rent. The bulk of the body is occupied by the bran- 

 chial sac, the mouth and all the viscera being collected into a small 

 space at the bottom. If the test were removed and a Mya-shell placed 

 over the inner mantle, the creature might pass for a Lamellibranch. 

 But there are no true gills ; the respiration being performed by the 

 more or less wrinkled lining of the water chamber : there is no foot : 

 the mouth has no lips to choose its food : there is no complete circu- 

 lating system ; the blood being carried backwards and forwards along 

 the same vessels ; and the reproductive functions are of so low an 

 order that fresh individuals can be produced by budding, as in plants. 

 The Ascidians are always fixed at the bottom of their squirts, and 

 may often be gathered on the fronds of sea-weeds, shells, &c. In 

 many places they are taken to market, and even considered dainty 

 articles of food. The Ascidia vary from one to six inches in length, 

 and often are brilliantly colored within, Molgula and Glandula have 

 globular bodies, differing in the number of lobes at the apertures. 

 Cynthia has a basket-shaped body, with two ovaries ; Dendrodoa has 

 only the left, and Pandocia the right ovary. Pera has a pear-shaped 

 body, scarcely adhering. Peloncea has a long body, ending in the 

 two pipes, and looks like the outside portion of a Panopcea. Chely- 

 osoma is a Greenland form, with a tortoise-shaped body. Boltenia is 

 kidney-shaped, resting on a long stalk, on which the young ones some- 

 times grow. 



Family CLAVELLINID/E. (Social Ascidians.) 



Here, for the first time as we descend downwards in the animal 

 scale, we meet with several living creatures, each having their own 

 organs of individual life, but all connected together into a common life 

 by prolongations from a central stem or creeper, in which the common 

 blood keeps circulating in opposite directions. The compound creature 

 is called a Zooid. The creatures are quite transparent, and very 

 small. New creatures are formed by buddings-off from the common 

 stem, as well as by fresh eggs. Clavellina looks like a bunch of Cine- 

 ras. Perophora grows on sea-weed, like little specks of jelly dotted 

 with orange and brown. Syntetliis grows in dahlia-shaped masses 

 six inches across. The zooid of Chondrostachys has a long cylindrical 

 stem. 



Family BOTRYLLIDJE. (Compound Ascidians.) 



These creatures have their tests fused into a common mass, so that 

 each zooid looks like a single animal outside ; but the individuals are 

 found to be separate within. In the Botryllians, the individuals are 

 united into systems round common excretary cavities. In the Didem- 

 niansy the chest and abdomen are distinct. In the Polyclinians, there 

 is a chest, with the breathing organs ; an upper abdomen, with the 

 digestive organs ; and a lower abdomen, with the heart (so called) and 

 reproductive organs. 



In Botryttus, the breathing-holes are star-shaped, the cloaca being 

 poured into a common sewer. In Botrylloides, the stars are more 

 irregular, and the animals are vertical. 



The zooid of Didemnium is very irregular, the individuals with a 

 pedunculate abdomen. In Euccdium, the animals are scattered, or 

 arranged in quincunx. Leptodinum makes thin, variously colored 



