LECTURES ON MOLLUSC A. 277 



Family Ascidiad^. {Sea-Squirts.) 



The Sea-squirts appear at first sight nothing but leathery bags, 

 covered perhaps with sea-weed or other accretions. The presence of 

 organic life is only made known to us by the violent jets of water 

 which they force out when disturbed. This leathery bag or "test" 

 takes the place of the shell in the bivalves. It is less distinctly ani- 

 mal in its nature than any other substance produced by sentient life, 

 containing a large quantity of the vegetative cellulose. It is freely 

 bored into by bivalve mollusks, such as Crenella and Mytilimeria. But 

 under this test, is found a delicate mantle, like that of ordinary mol- 

 lusks, united into a sac, and terminating in two openings,, the inha- 

 lent and excurrent. 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-weecls, 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. Pelonoea 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 Clavellinidjj;. (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. Glavellina looks like a bunch of Cine- 

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

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

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

 stem. 



