522 THE ANATOMY OF IXYERTEBRATED ANIMALS. 



and the walls of the branchial sac, coalesce and become per- 

 forated, m order to give rise to the stigmata. 



The test appears, at first, to be a cuticular secretion of 

 the epiblast, and to derive its cellular elements from the wan- 

 dering into its substance of cells derived from the epiblast. 



In 3Iolgula tubulosa^ Kupfer and Lacaze-Duthiers have 

 observed that the fecundated eggs are expelled from the 

 atrial cavity, and almost immediately become fixed to the 

 surface on which they fall. Yelk-division takes place, and, 

 after four nearly equal blastomeres are formed, much smaller 

 ones are developed from one face of these, and increase until 

 they constitute a blastodermic layer around the larger blasto- 

 meres, which undergo a slower division. The alimentary 

 cavity is formed by invagination. The embryos leave the 

 egg as voal bodies, capable of undergoing considerable but 

 slow changes of form, and devoid of any caudal appendage. 

 Each embryo rapidly invests itself with a transparent test, 

 throws out several tubular prolongations of the ectoderm, 

 and finally passes into the adult condition. Although no tail 

 is developed, a cellular mass is to be seen in the same posi- 

 tion as that occupied by the remains of this appendage, when 

 it has undergone its retrogressive metamorphosis, in the As- 

 cidians with caudate larvae. The atrial aperture is single at 

 its first appearance, and no larval sensory organs are devel- 

 oped. 



In the compound or social Tuyiicata, many ascidiozoSids, 

 which are united by a common test into an ascidiarium, are 

 produced by gemmation from a solitary metamorphosed larva. 



Sometimes, as in Clavellna and Perophora^ the parent 

 ascidiozo(3ids give rise to creeping stolons, from which branches, 



^le atrium of the adult. Kowalewsky, Fol, and later observers, afrree tliat 

 these openinsrs and the atrial sacs are formed hy two involutions of the ecto- 

 derm, which applv themselves to the sides of the pharynx, and coalesce with 

 it at the points which become perforated by the stigmata; of which, in Plial- 

 lusia, there are at first but two on each side. If this is a true account of the 

 oricrin of the atrium, the atrial membrane is obviously part of the ectoderm, 

 and its cavity is analo,'Xous to the pallial cavity of a mollusk. 



On the other hand, MetschnikofF and Kowalewsky agree that in the buds 

 of Botri/llus, and other ascidians which multiply by gemmation, the two primi- 

 tively distinct atrial cavities are portions of the alimentary sac, which become 

 shut off from it, and subsequently open outward. 



Metschnikoff (" Entwickeluriesgeschichtliche Beitrage." "Bulletin de 

 I'Acad, St.-Petersbourg," xiii.) therefore compares the atrium to the entero- 

 co^le of Echinodcrms. Renewed observations specially directed to this point, 

 which is of great morphological importance, are much needed. If the atrial 

 cavitv is really an enterocoele, it will answer to the perivisceral cavity of the 

 Brachiopoda., the pseudo-hearts of which will correspond with the primitive 

 atrial aperture. 



