26 £. 8. Morse— Classification of Mollusca 
openings i rm in fig. 1 by arrow O) follow the same di- 
rection, that rom po osterior to anterior, ventrally. Thus in 
Tunicata the vied openings are posterior ‘and posterior-dorsal ; 
the posterior-dorsal, being the anal or excurrent orifice; this i is 
always the shortest in Tunicata. In Lamellibranchiata the anal 
tube moves nearer the branchial tube; in the lower forms their 
outer covering coalescing and of equal length, while, Hamed up, 
the tubes become entirely separate, and, in some, of extreme 
~ he anal tube being the longest. Tn Pisidiam and other 
forms the branchial tube disappears, and water is received 
decid a sivenkend opening; while the anal tube yet remains, oc- 
cupying a posterior position on a line with the antero-posterior 
, in the same position the branchial tube vawiiank in the 
Tan nicata : and, finally, both tubes become nearly obsolete, and 
the mantle is cleft a round, except dorsally. Thus the pro- 
gress of sac opening follows in the same line of rotation with 
an opposite “direction (fig. 1, arrow A). Commencing with 
the Polyzoa as the lowest’ = we have, as in the Cheilosto- 
little lid). This mode of attachment is es lowest feature; 
namely, attached along the entire dorsal re 
As we ascend to the higher forms of the ee we have a free- 
ing ot the posterior portion of 5, and the viscera perma- 
men springing anteriorly, and from t e valve, as in the 
partially freed polyzoon. Crania and ini are attached as 
in Lepralia. 
In Lingula, where we have the lengthened and flattened sac, 
the animal stands vertical in the sand. In Terebratula an 
genera, the dorsal valve already assumes preponderance. 
over the oe valve, and now obtains its normal position 
—— 
All ah Tunicates with few exceptions are attached, and by 
their anterior en 
n the compound Ascidians like rat bee where we have & 
donatnteniey of individuals clustering round a common center, 
their dorsal as well as anterior regions ate ——? or, in other 
words, oe ventral and or regions are free 
This a nece with importans excep 
