SALPIDAE FORMS WITHOUT COVERING FOLDS. 



431 



of the body (Fig. 216), but later, as the part of the body known as the 

 nucleus decreases in size, it shifts further back. 



The rudiment of the alimentary canal, in the strict sense of the 

 term (Fig. 214 7>, d}, originally forms a posteriorly directed diverti- 

 c ulum of the pharynx. This caecum becomes divided later by two 

 folds rising into it from below into three spaces (Fig. 216, oe, m, ed), 

 the anterior space being the rudiment of the oesophagus, while the 

 posterior space represents the intestine. The middle space is the 

 rudiment of the stomach-caecum. The intestine curves upward 



KK,. 21b'. Later embryonic stage of Salpa democratica-mucronata (after SALENSKY). 

 e, a trial aperture ; eb, elaeoblast ; ed, intestine ; es, endostyle ; ft, ciliated pit ; i, 

 branchial (oral) aperture ; k, gill ; m, stomach-caecum ; n, ganglion ; oe, oesophagus ; 

 //, pmcanUal sac : jtl, placenta ; st, stolon ; t, so-called roof of the placenta (basal 



plate). 



towards the left, till its blind end comes into contact with the 

 atrial wall, perforation leading later to the formation of the anal 

 aperture. 



It has been shown that the niesoderm (Fig. 213 B, ms) spreads out 

 over the right and left sides of the embryo in the form of two lamellae 

 in close contact with the entoderm. These lamellae, according to 

 LEUCKART, yield the muscle-hoops, a kind of fenestration taking place 

 in the lamellae and separating the mesoderm-bands which correspond 

 to the different hoops. The transversely striated contractile substance 

 develops later in the muscles. The heart also, according to SALENSKY, 



