[445] INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 151 



Fuller. It may, nevertheless, occur annually in winter, and yet be sel 

 dom observed ; for very few naturalists go out to collect marine animals 

 in winter and early spring. 



The bivalve shells mostly produce minute young, or larvae, which are 

 at first provided with vibrating cilia and swim free for several days, as 

 is well known to be the case with the oysters, clams, muscles, Teredo, &c. 

 But a few species, like the Tottenia gemma, (p. 359,) produce well dvel- 

 oped young, furnished at birth with a well formed shell. 



The common fixed Ascidians, both simple and compound, mostly pro 

 duce eggs that hatch into tadpole-shaped young, which swim about for 

 a short time by the undulatory motions of the tail, but finally become 

 fixed by the head-end, and losing, or rather absorbing, the tail-portion, 

 rapidly develop into the ordinary forms of the ascidians. This pro 

 cess, although often very rapid, is a very interesting and complicated one 

 In Molgula Manhattensis there is, according to the observations of Dr. 

 Theodore A. Tellkampf, an alternation of generations. He states that 

 the minute yellow ova were discharged July 18, invested in a viscid 

 yellowish substance, which become attached to the exterior of many 

 specimens. In a few days the &quot; viscid substance&quot; had changed its ap. 

 pearance and became contractile; the ova became larger, round, and of 

 different sizes ; &quot; after two or three days the largest protruded some 

 what above the surface of the common envelope, and presented a circular 

 or oval aggregation, like that of the Mammaria found a year ago ? on 

 the llth day, the round ova had increased in size, with a central round 

 or oval orifice through which the motion of the cilire of the branchial 

 meshes were visible. &quot; The orifice had approached on the 1st of August 

 more or less to one apex; in some specimens, which were now oval, it 

 was terminal.&quot; In this stage he names it Mammaria Manhattensis, 

 regarding the Mammaria as a &quot; nurse ;&quot; within, each of the Mammarice, at 

 the end opposite the branchial orifice, there was seen a mass of cells, 

 which ultimately developed into a tadpole-shaped larva, similar to that of 

 other ascidians. He observes that the Mammarice increase after the 

 discharge of the Iarva3, and that gemmation takes place within the 

 common envelope.* These observations, if correct, are very interesting 

 and important, but they need farther confirmation. The development 

 of the larvae from the Mammarice into Molgula was not traced j neither 

 did he witness the actual discharge of the ova, which produced the 

 Mammarice, from the Molgula. They may possibly have no relation with 

 one another. 



Several kinds of Ascidians, however, swim free in the water during 

 their entire life. The most common Ascidian of this kind is the Salpa 

 Cabotti, (Plate XXXIII, figs. 254, 255.) This, like the other species, 

 exists under two different forms ; or, in other words, it is one of those an 

 imals having alternations of generations. The sexual individuals (fig. 

 255) are united together into long chains by processes (c) from the sides 



* Annals of the Lyceum of Natural History of New York, Vol. 10, p. 83, 1872. 



