522 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



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

 forated, in 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 Molgula 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 Tunicata, many ascidiozooids, 

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

 produced by gemmation from a solitary metamorphosed larva. 



Sometimes, as in Clavelina and Perophora, the parent 

 ascidiozooids give rise to creeping stolons, from which branches, 



gle atrium of the adult. Kowalewsky, Fol, and later observers, agree that 

 these openings and the atrial sacs are formed by two involutions of the ecto- 

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

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

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

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

 and its cavity is analogous to the pallia! cavity of a mollusk. 



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

 of Botryllus, and other ascidians which multiply bv 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 (" Entwickelunsrsgeschichtliche Keitrage," "Bulletin de 

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

 coele of Echinoderms. " Renewed observations specially directed to this point, 

 which is of gi*eat morphological importance, are much needed. If the atrial 

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

 BracMopoda, the pseudo-hearts of which will correspond with the primitive 

 atrial aperture. 



