154 ^ EMBRYOLOGY 



there fuse ; thus the arrangement of the vessels of the adult animal is 

 reached. 



The metamorphosis of the Beroidce described by Allman (No. 2) and 

 A. Agassiz (No. 1) takes place in an unusually simple manner. The larva 

 is at first round in cross-section, and later is flattened transversely. Of 

 the meridional vessels the subsagittal first grow the more vigorously, and 

 reach the edge of the mouth, where they meet two processes from the 

 gastral vessel of the same side, which extend along the edge of the mouth, 

 and fuse with them. The subtransverse vessels meet these transverse 

 processes only later, and then the ramifications of the vessels begin to 

 grow forth. 



General Considerations. — The Ctenophora exhibit in 

 their organization a whole series of features by means of 

 which a closer relationship with the Cnidaria — Coelenterata 

 in the narrowest sense — appears to be established. To this 

 series belong, if we ignore the more superficial resemblances 

 of the gelatinous transparent body, first of all the possession 

 of a very similar gastrovascular system, the presence of 

 tentacles, the bases of which exhibit relations to the canals 

 of this system, the position of the ripening sexual products 

 in these canals, and the similar character of the eggs. In 

 fact, up to the present time the Ctenophora have usually 

 been incorporated with the Coelenterata; Haeckel (No. 11), 

 indeed, with whom Chun (No. 8) concurred, conjectured 

 that, the group of the Cladonemidae, and particularly 

 Ctenaria, which belongs to this group, constituted the inter- 

 mediate link between the Anthomedusae and Ctenophora. 

 Even though this genus presents a remarkable resemblance 

 to the Ctenophora in the possession of only two marginal 

 tentacles and corresponding caecal pouches, in the mesogloea 

 of the umbrella (tentacle-sheaths), and in the eight ex- 

 umbrellar nettling ridges corresponding to the ribs, still the 

 view that these resemblances were based upon true homology 

 has been somewhat shaken by Hartlaub (Nos. 10 and 11), 

 who was able to produce evidence that in the closely allied 

 Eleutheria the brood-cavity lying over the stomach arises as 

 ^n ectodermal invagination from the cavity of the umbrella, 

 and therefore could not be homologized, as Haeckel main- 

 tained, with the infundibulum of the Ctenophora. Even 

 earlier than this R. Hertwig (No. 12, p. 444^ had produced 



