636 



INVERTEBRATA 



CHAP. 



mesoderm. A posterior mass is the rudiment of the elaeoblast, a 

 protuberant mass of fatty globules which probably represents the last 

 vestige of the lost larval tail. The pharyngeal cavity then appears 

 in the centre of the median portion of the endodermic mass. It is 

 nearly divided into two by a median dorsal infolding, the rudiment 

 of the adult "gill." The cloacal cavity appears as a median invagina- 

 tion of ectoderm, and unites with the pharynx at the sides of this 

 so-called " gill." 



The nervous system separates from the ectoderm and becomes 

 hollowed out, showing a transient indication of division into three 



mas c cure 



m 



FIG. 462. The embryo of Salpn fusiformis attached to the maternal placenta, seen from 



the side. (After Salensky.) 



a, anus ; at, atrial opening ; d.l, dorsal lamina (remnant of dorsal wall of pharynx) ; el, elaeoblast ; 

 endst, endostyle ; H, heart; h.j>, blood-forming organ in the placenta; //.i/yi, hypophysial canal; hit, 

 intestine ; m, mouth ; n/si.-.oi/r, circular bands of muscle ; n.p, nerve ganglion ; UP.S, oesophagus ; per, 

 pericardium ; pi, placenta. 



chambers, which have been compared to the three brain-vesicles of 

 the embryos of higher Vertebrates (n.t, Fig. 464). The, neural tube 

 becomes connected with the pharynx close to where the mouth is 

 formed as an invagination. The mesodermic mass on each side gives 

 rise to the characteristic hoop-like muscles which are the adult organs 

 of locomotion. The elaeoblast is now gradually absorbed, the embryo 

 wrenches itself loose, breaking off the placental knob and carrying 

 away with it this portion of the maternal tissues embedded in its 

 body, where it is gradually absorbed, though its remains can be 

 recognized for a long time. 



In Dolioluni, the other well-known genus of Thaliaceae, the egg 

 is shed into the sea, and its early development seems to resemble that 



