754 THE ANIMAL KINGDOM. 



The ecto- and endo-derm cells grow each in a single layer round the 

 colossal cell, leaving a slit-like or triangular opening by which it protrudes 

 its pseudopodia. The elongate or triangular larva now swims about and 

 settles eventually on the gastric peduncle of its host. It begins to bud, 

 whilst the colossal cell atrophies, and is then transformed into a clavate sac, 

 not quite \ in. long, with walls composed of an ectoderm, longitudinal ecto- 

 dermal musculature, a supporting lamina, and an endoderm with muscle 

 fibres (? circular). This sac, when full-grown, carries buds in numbers up to 

 about i oo, which develope into eight-rayed Cttmnae,and are detached. The 

 cycle therefore includes an Alternation of Generations, and Metschnikoff 

 has suggested that there may be three generations, (i) a true Cunina, (2) 

 a sac-like larva (supra], and (3) a Cunina-like Medusa with peronia but no 

 festoon-canals. 



As to the Trachomedusae, Aglaura hemistoma has an elongate larva 

 with an axis of endoderm cells in a single row. The mouth is formed at 

 one pole, and near it the first two tentacles, and then at right angles to 

 them two more. In the Geryonidae mesoglaea makes its appearance as 

 soon as the endodermal sac is complete. A patch of thickened ectoderm 

 is then formed at one spot and the endoderm sac gradually approaches and 

 at last comes into contact with it, becoming, sooner or later, flattened. The 

 edge of the patch thickens, while its centre becomes thin. The primary 

 radial solid tentacles (four in Liriope, six in Geryonia] appear in connection 

 with the thickened edge which gives origin to the margin of the umbrella 

 and velum, whilst the central disc becomes the ectoderm of the subumbrella 

 and manubrium. The mouth breaks through in its centre. The interradial 

 solid tentacles, in number equal to the radial, next appear. During growth 

 the Medusa undergoes changes. The oral peduncle grows down, carrying 

 the manubrium with it ; the hollow radial tentacles of the adult spring from 

 the margin of the bell, whilst the solid radial tentacles drop off; the solid 

 interradials are lost in some genera, not in others 1 . 



The hydroid appearance of the larva is much masked in the Geryonids 

 by the early development of mesoglaea, in Cunoctantha parasitica by 

 parasitism. In other instances, especially in C. octonaria, it is exceedingly 

 well-marked, and it has a resemblance to the Actinula larva of certain 

 Hydroidea (p. 765). 



The Trachymedusae are marine, unless the freshwater Limnocodiuwt, of 

 unknown habitat, and a Medusa, lately found in Lake Tanganyika (Central 

 Africa), belong to the order 2 . Some of them have been dredged at great 



1 The solid radial and interradial tentacles differ from one another in shape, &c. Brooks states 

 that in Liriope scutigera the interradial tentacles are lost or retained irregularly. Hence the point 

 is not, at least not always, as Haeckel considers it to be, of diagnostic value. See Mem. Boston 

 Soc. Nat. Hist. iii. p. 383. 



2 By Allman Limnocodium was placed among Leptomedusae. There are three^ points about 



