832 



MANUAL OF THE MOLLUSGA. 



resemblance to a water-skin, or small leather bottle {ascidium). They 

 attain a length, of several inches, and are fixed to rocks or shingle, or sea- 

 weed, but sometimes so slightly that they are brought up detached, and yet 

 uninjured, by the dredge. Their appearance is sufficiently unpromising; 

 their surface often rugged or concealed by adhering sand and fragments of 

 shell ; sea- weeds grow upon them, and small bivalves {crenella) burrow in 

 their tunic. They are hollow and elastic, and have two orifices, from which 

 (especially the terminal opening), they squirt water, as the bivalve shell-fish 

 do when molested. 



If the soft outer shell (^0 is opened 

 there will be found inside a second tunic 

 {t) which is compared to the mantle of 

 the bivalves ; it is extremely muscular, 

 the fibres circling round it closely, espe- 

 cially near the orifices, whilst some others 

 are oblique and longitudinal. The mantle 

 lines the tunic, but is only slightly at- 

 tached to it at the two orifices, and at 

 those points where the blood-vessels pass 

 through,* 



During life the outer tunic follows the 

 contractions of the muscular mantle ; and 

 when the latter relaxes, the tunic returns 

 to its original shape by virtue of its 

 elasticity. But when preserved in spirit 

 the mantle contracts to such an extent 

 as to tear itself away from the tunic, and if 

 such a specimen is opened the muscular sac 

 looks like a little tunicary quite loose 

 Fig, 224, Ascidian.t within the large one. Within this a 



third and more deUcate tunic is formed by the respii-atory or branchial sac 

 {b) having only one external orifice by which it is suspended, a little within the 

 terminal "(or exhalent) opening of the outer tunics ; as its texture is porous the 

 water passes through it readily into the mantle cavity, and thence by the second 

 * In the thick pellucid test oi Ascidium mamillatum the eye can discern an exten- 

 sive network of vascular ramifications. The bloodvessels enter the test near the 

 base. In the closely allied genus Cynthia there is no such vascular connexion, but 

 the mantle is more strongly united to the test at the orifices ; in Chelysoma the tunics 

 are extensively united by muscular fibres. (Rupert Jones ) The relation between the 

 Ascldian test and mantle is that of the epidermis to the cutis vera, precisely as in the 

 laniei;ibrancliiate l,ivalves; the union of the two in the majority of Ascidians is ex- 

 ceedingly intimate in tbe fresh state. {Huxley.') 



+ Fig. 224, Ascidium vionachus ; 'in. incurrent; ex. excurrent orifice; t'. outer 

 unic ; t. niuf ciilar tunic ; b. branchial sac ; o. tentacular fringe ; ff. nervous ganglion ; 



