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 MoUjula tuhulosa^ Kupfer and Lacaze-Duthiers have 

 observed that tne 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 bJastomeres are formed, much smaller 

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

 they constitute a blastodermic laj^er around the larger blasto- 

 meres, which undergo a slower division. The alimentary 

 cavity is formed by invagination. The embryos leave the 

 Q^^ as voal bodies, capable of undergoing considerable but 

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

 Each embryo rapidly invests itself witli 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 larvpe. The atrial aperture is single at 

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

 oped. 



In the compound or social Timicata^ many ascidiozooids, 

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

 produced by gemmation from a solitary metamorphosed larva. 



Sometimes, as in ClaveUna and Perojohora^ the parent 

 ascidiozooids give rise to creeping stolons, from which branches, 



Erie atrium of the arlnlt. Ko"vrale"n's1\:y. Fol, and later ol-)i=!ervers, agree that 

 these oponniofs and the atrial saes are fonned by two involutions of the ecto- 

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

 it at the points which hecome perforated hy the stisfmata; of which, in Plial- 

 luf^ia^ there are at first but two on each side. If this is a true account of the 

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

 and its cavity is analo,^ous to the pallial cavity of a mollusk. 



On the other liand, Metschnikoff and Kriwalewsky aarree that in the b-ads 

 of Botrylhis^ and otlier 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 (" Entwickelunesfjeschichtliche Beitrnsre." "Bulletin de 

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

 Cff-le of Echinodcrms. Renewed observations specially directed to this point, 

 which is of irreat morpholocrical importance, are much needed. If the atrial 

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

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

 atrial aperture. 



