730 THE ANIMAL KINGDOM. 



they are present only from place to place in pairs, 5-7 to each half of the 

 body. 



The Beroidae are conical or ovate in shape, a good deal broader in the 

 stomachal than in the funnel plane. Nets cordigera has two large lateral 

 lobes prolonged aborally. Tentacles and tentacular vessels are absent even 

 in development. The central nervous system is freely exposed, i. e. not 

 counter-sunk in the body, and the edges of the elliptical polar- plates are 

 produced into branched villiform processes. The mouth is of great size 

 and expansible : the stomach voluminous, and its oral end armed with 

 sabre-shaped cilia-plates which prevent the escape of the prey. There are 

 no stomachal ridges (p. 723) : the funnel is small and there is no vertical 

 funnel vessel, but the two excretory vessels spring separately from the 

 funnel, and the blind ampullae are extended to a great length beneath the 

 polar areae. The perradial vessels are abortive ; the vessels are large, the 

 paragastric especially so : the latter divide at the oral margin into a right 

 and left horizontal vessel, not united as generally stated into a circular 

 canal. Into these horizontal vessels fall the corresponding meridional 

 vessels. Both the paragastric and meridional vessels give off lateral 

 branches. The latter traverse the mesoglaea in all directions in Beroe ovata. 

 The branches of the sub-tentacular and sub- ventral vessels of the anterior 

 half of the body unite inter se as well as with the paragastric branches : so 

 too the corresponding branches of the posterior half of the body, but there 

 is no lateral connection between the anterior and posterior networks in 

 B. ovata as there is in B. Forskalii, and more especially in Nets cordigera. 

 Idya has no branches from the paragastric vessels. The sexual products 

 are placed at the sides of the meridional vessels and more or less in their 

 lateral branches, especially in B. Forskalii'. in B. ovata and Idya they 

 extend along the canals nearly to the oral margins *. 



The egg of Ctenophora is suspended in a large mass of jelly inclosed 

 by a delicate membrane. As to the development the gastrula is epibolic : 

 and the gastrula mouth is said by Chun to correspond with the aboral pole. 

 The mouth and stomach are formed by an invagination of the ectoderm : 

 the endoderm giving origin to the funnels and the various vessels. The 

 muscular fibres in the mesoglaea are said to be derived from immigrant 

 ectoderm cells both superficial and stomachal 2 . The ectoderm is at first 



1 In Nets cordigera the genital products are confined exclusively to the network of vessels. But 

 von Lendenfeld believes that they are derived from cells of the endoderm (?) of the meridional vessels 

 which withdraw into a layer of sub-epithelial cells present in this Beroid, through which they wander 

 to the spot where they ripen. 



2 This process of immigration may continue during life, according to Chun (op. cit. infra, p. 

 197). Metschnikoff denies the accuracy of Chun's and Kowalewsky's observations. His conclusions, 

 briefly put, are as follows : (i) The embryonic epiblast forms a ring of cells round the cells of the 

 hypoblast, leaving a pore at each end of the principal axis, the blastopore and pseudo-blastopore. 

 (2) A mass of small ' mesoderm ' cells is formed from the hypoblast cells at the blastopore. (3) A 



