432 PYROSOMID^l. 



not unfrequently spreading over the larger isolated species of 

 Tunicata. They agree with them, also, in the relative positions 

 of the branchial orifice and the funnel, and in the structure of the 

 respiratory chamber. The individuals are occasionally connected 

 by a gelatinous flesh, like that which exists in many of the 

 compound Polypes ; and there is even sometimes found a calca- 

 reous deposition in this connecting substance which would still 

 more closely establish their affinity with that group. It has been 

 recently shown, also, that all the individuals in these compound 

 masses originate by gemmation, or budding, from a single one. 

 Both in the solitary and compound Ascidians, the young animal, 

 when it first issues from the egg, has active powers of locomo- 

 tion, being provided with a large tadpole-like tail, by the stroke 

 of which it is propelled through the water. Thus a provision is 

 made for the general diffusion of these animals, which would be 

 otherwise crowded in particular spots. 



1065. The PYROS^MID^: form a third family of compound 

 Ascidians, differing however from the. preceding forms, both in 

 the structure of the colony and that of the individual animals 

 composing it. The colony, which is unattached, forms a hollow 

 cylinder, from five or six to fourteen inches in length, open at 

 one end, and composed entirely of minute animals of peculiar 

 form. They differ from the other Ascidians in having the in- 

 current and excurrent orifices at the two extremities of the body, 

 and thus resemble the animals of the following Order, from 

 which they differ however in the structure of their branchiae, 

 which are truly Ascidian. These are aggregated together in 

 the following manner. A number unite, like the radii of a star, 

 so as to form a circle with a central space, into which the vents 

 of all the individuals open, their mouths being at the circum- 

 ference. Several of these circular clusters are piled one on an- 

 other, the central aperture of all of them corresponding so as to 

 form a tube ; and through one end of this tube a constant cur- 

 rent of water is forced out by the united ciliary action of all the 

 individuals, which is supposed to cause the mass to move 

 through the ocean in an opposite direction. According to recent 

 observations, however, it would appear that the Pyrosoma floats 



