INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC, 445 



Fullor. It may, nevertheless, occur annually in winter, ami 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 larv.ne, 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 devel- 

 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 tad, but finally become 

 fixed by the headend, 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 Mohjulti Manhatteasis 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 " viscid substance" had changed its ap. 

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

 different sizes; " 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 11th day, the round ova had increased in size, with a central round 

 or oval orifice through which the motion of the cili;i^ of the branchial 

 naeshes were visible. " 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." In this stage he names it Mammaria ManhatfensiSj 

 regarding the Mammaria as a " nurse ;'' within each of the Mammariw, 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 Mammaria} increase after the 

 discharge of the larvre, and that gemmation takes idace within the 

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

 and important, but they need farther confirmation. The development 

 of the larvie from the Mammaria; into MoUjula was not traced; neither 

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

 Mammarim, 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 

 Cahotti, (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 tlie Lyceum of Natural History of Ncav York, Vol, 10, p. 83, 187:2. 



