CTENOPHORES OF THE ATLANTIC COAST OF NORTH AMERICA 29 



-ectoderm, while others lie at first at the lower (oral) pole of the embryo 

 but are carried inward by invagination until they come to lie immediately 

 under the apical sense-organ, where they give rise to the mesoderm. 

 The stomodasum is formed by invaginated ectoderm derived from 

 the original macromeres (figs. 26, 27, plate 6). The axial funnel-tube and 

 peripheral chymiferous vessels are of entodermal origin and are formed 

 from the macromeres after the formation of the primitive mesodermal 

 ■cells. The body becomes egg-shaped, the aboral pole being broadest. 

 There is considerable lateral compression, the tentacular axis being wider 

 than the sagittal. When the animal is mature, however, the reverse is 

 the case, for then the tentacular diameter is the narrower. The combs 

 of cilia appear as 4 double rows of simple lashes. These soon separate, 

 however, forming 8 rows of ciliated plates. The apical sense-organ is at 

 first situated within a very shallow depression at the aboral pole, but 

 this cleft gradually deepens until the organ becomes stmken deeply 

 within a furrow. When about 30 hours old, the embryo has acquired 

 4 double rows of cilia, a well-developed pair of lateral tentacles, and a 

 large, apical sense-organ (fig. 34, plate 6). The entodermal part of the 

 gastro-vascular system consists of 6 lateral diverticula from a central 

 •chamber; 2 of these lateral branches lead into the bases of the tentacles 

 and the other 4 lead outward toward the 4 double rows of cilia. The 

 ectodermal buccal pouch, or stomodaeum, has become a long, laterally 

 compressed tube, with its broad axis 90° from the tentacular axis of the 

 animal. Until this time the animal swims about quite freely within the 

 egg-envelope, and even in this stage its cilia may be observed to beat in a 

 normal manner and the tentacles to elongate or contract in response to 

 stimuli. Soon after this the larva breaks through the egg-envelope and 

 escapes into the water. Here it goes through stages which are so close 

 to those of the young Pleurobrachia that it is almost impossible to dis- 

 tinguish the embryos of the two species apart. The tentacles acquire 

 numerous lateral filaments and elongate greatly, as in Pleurobrachia. 

 When the animal is 5 mm. long the oral lobes begin to develop as two 

 simple outgrowths on both sides of the mouth in the sagittal plane of 

 the animal (see fig. 39, plate 6). The sagittal diameter is then somewhat 

 longer than the tentacular diameter, and from this time onward the dis- 

 parity between these two diameters increases at the expense of the 

 tentacular diameter. At the time when the oral lobes begin to develop, 

 the meridional ventral canals and the paragastric tubes begin to elongate 

 downwards (fig. 39, plate 6). The former give rise to the characteristic 

 loops in the oral lobes, while the 2 gastric tubes become X-shaped at 

 their ends and finally extend up the lateral furrows which lie adjacent 

 to the sides of the lobes, forming a circumoral vessel around the lobes. 

 The 4 meridional vessels extend downward and fuse with the circum- 

 oral vessel. The primary tentacle-bulbs migrate downward at the same 

 time and come to lie close by the sides of the mouth, and they carry 

 the tentacular canals down with them. The auricles appear last of all, 

 after the lobes have developed to some extent. When 10 mm. in length 

 the animal is ellipsoidal in outline and the condition of the lobes and 

 auricles is similar to that in the adult of Bolinopsis (see fig. 40, plate 7). 



