398 



ZOOLOGY. 



of several species of Molgula, but it has the five long decid- 

 uous appendages observed in young MolgulcB. Among the 

 compound Ascidians, Botryllus and Botrylloides have tailed 

 young, while in other forms there is no metamorphosis, de- 

 velopment being direct. 



Order 3. Thaliacea. — Oil the whole, we may regard this 

 order, represented by Salpa (Fig. 386°), and Doliolum, as 

 comprising the more specialized forms of Tunicates. Salpa 

 is pelagic, one species occurring in abundance ofE the shores 



of Southern New England, while 

 the others mostly live on the high 

 seas all over the tropical and sub- 

 tropical regions of the globe. Late 

 in the summer our Salpa spinosa 

 of Otto can be captured in multi- 

 tudes by the tow-net in Long Island 

 Sound. 



There are in Salpa two kinds of 

 individuals, i.e:, the solitary, and 

 aggregated or chain-Salpse. The 

 body of the solitary or asexual 

 form is more or less barrel-shaped, 

 with a series of circular bands of 

 muscles, like the hoops of a barrel, 

 and situated on the inner side of 

 the outer tunic. The test is trans- 

 parent, though very thick, while 

 the outer tunic lines the cavity of 

 the test as in other Tunicates. In 

 the members of this order the oral 

 aperture of the mantle is at one 

 end of the body, and the atrial 

 opening at the opposite end, the minute digestive canal be- 

 ing but slightly curved, the body-cavity being largely occu- 

 pied by the pharyngeal or respiratory sac. Moreover, the dor- 

 sal or hasmal side of the body is clearly distinguishable from 

 the ventral or neural side, as well seen in Doliolum, where 

 the well-marked tubular heart lies above the digestive organs, 

 and is directly opposed, as in worms generally, to the nervoua 



Wg. 386'.— SaZpa spinosa. An 

 individual from a mature chain; 



ttiree- quarter view, enlarged, a, 

 atrial opening ; b, moutli ; c, pro- 

 ce-^Bes l)y which the members of 

 the chain are united ; A, heart; n, 

 nervous ganglion ; o, nucleus ; r, 

 gill.— After A. Agassiz, fromVer- 

 rill's Report. 



