COMPOUND ASCIDIANS. 



501 



FIG. 248. 



discerned without dissection, but allows many of their living ac- 

 tions to be watched. Of these we have a charac- 

 teristic example in Amaroucium proliferum ; of 

 which the form of the composite mass and the 

 anatomy of a single individual are displayed in 

 Fig. 248. The clusters of these animals pre- 

 sent themselves on rocks, sea-weeds, &c., very 

 commonly between the tide-marks. They ap- 

 pear almost completely inanimate, exhibiting 

 no very obvious movements when irritated ; but 

 if they be placed when fresh in sea water, a 

 slight pouting of the orifices will soon be per- 

 ceptible, and a constant and energetic series of 

 currents will be found to enter by one set and 

 to be ejected by the other, indicating that all 

 the machinery of active life is going on within 

 these apathetic bodies. In. the tribe of Poly- 

 cliniansj to which this genus belongs, the body 

 is elongated, and may be divided into three 

 regions, the thorax (A), which is chiefly occupied 

 by the respiratory sac, the abdomen (B), which 

 contains the digestive apparatus, and the post- 

 abdomen (c), in which the heart and generative 

 organs are lodged. At the summit of the 

 thorax is seen the oral orifice, c, which leads to 

 the branchial sac, e; this is perforated by an im- 

 mense number of slits, which allow part of the 

 water to pass into the space between the bran- 

 chial sac and the muscu- 

 lar mantle, where it is 

 especially collected in 

 the thoracic sinus,/. At 

 k is seen the oesophagus, 

 which is continuous with 

 the lower part of the XNJ* 

 pharyngeal cavity ; this (^Iw /?;> 

 leads to the stomach, ?, 

 which is surrounded by 

 biliary tubuli ; and from 

 this passes off the intes- 

 tine, m, which termi- 

 nates at n in the cloaca. 



The long post-abdomen of integtine in cloaca; o heart . ^ pericardium . Pt ova . 



Compound mass of Amaroucium proliferum, with the 

 anatomy of a single zooid: A, thorax; B, abdomen; c, 

 post-abdomen : c, oral orifice ; e, branchial sac ; /, tho- 

 racic sinus; i, anal orifice ; i', projection overhanging it; 

 j, nervous ganglion ; .k, resophagus ; I, stomach sur- 

 rounded by biliary tubuli; m, intestine; n. termination 



is principally occupied 



by the large ovarium, 



P, which contains ova 



in various stages of development. These, when matured and 



set free, find their way into the cloaca ; where two large 



ova are seen (one marked p' and the other immediately below 



rium ; p', egg ready to escape ; q, testis ; r, spermatic 

 canal : r', termination of this canal in the cloaca. 



