752 THE ANIMAL KINGDOM. 



developed on the radial canals, in the Aglauridae alone sometimes at or 

 near the base of the manubrium. They either occupy the whole sub- 

 umbrellar wall of the canals, or are divided by radial muscles into two 

 halves. They have a narrow base in the Aglauridae, and depend into the 

 bell cavity : and in the Geryonidae they are leaf-like, the apex of the leaf 

 being turned towards the bell-margin l . 



The ovum is naked, as it always is in Craspedote Medusae. The 

 smallest Medusan ovum_known is that of Cunina proboscidea ('024mm.), the 

 largest that of Polyxenia (Solmisstts) albescens (1.5 mm.). Segmentation is 

 regular ; in Polyxenia (Solmoneta) flavescens var. leucostyla variably irregular; 

 in Aglaura hemistoma irregular. There is a solid morula, or in Geryonidae 

 a blastula. The endoderm in the latter is derived by primary delamination, 

 i.e. transverse fission of the cells: in the former in various ways ; (i) by 

 immigration of cells at any point, Aeginopsis (Solmundella) mediterranea ; 

 (2) by secondary delamination, i. e. by arrangement of the cells in two 

 layers, Aglaura hemistoma^ the Trachynemid Rhopalonema velatum ; or (3) 

 in a mixed mode, primary or secondary delamination, immigration, Polyxenia 

 leucostyla. 



In the Narcomedusae the further development of some Solmaridae, and 

 the life history in part of some Cunanthidae have been traced. As to the 

 former, Solmundella ^.Aeginopsis mediterranea, Solmoneta s. Polyxenia flaves- 

 cens, var. leucostyla, have an elongated ciliated free-floating larva the two 

 ends of which become transformed into the two primary tentacles, the 

 central region acquiring a mouth. In the latter of the two named, two 

 tentacles have been observed developing at right angles to the two first, 

 and a tentaculocyst appearing in each interval between the tentacles. 

 Later stages with eight and twelve tentacles have been seen, but the 

 number of tentaculocysts present was not constant. The central part of 

 the bell is produced by the growth of the aboral part of the body within the 

 circlet of tentacles : the peripheral part by the growth of a ridge in the 

 same zone with the tentacles or to their oral side. But the development of 

 this region has been more accurately traced in some Cunanthids. The 

 lobes are produced by the more rapid growth of the parts between the 

 tentacles ; the peronia by the union of the ectoderm fringing the outer 

 surface of the margins of adjacent lobes, whilst the velum appears to be an 

 outgrowth of their inner or oral aspect. The segmentation of the egg in the 

 Cunanthidae has never been observed. The life histories of several mem- 

 bers of the family, so far as known, are most remarkable, (i) Cunoctantha 

 (Cunina) octonaria, parasitic in. the bell of the Anthomedusan Turritopsis. 

 The first noted stage (? developed from an ovum) is proboscidiform with a 



1 A female specimen of Geryonia hexaphylla ( = Carmarina fungiformis, Haeckel, ' System/ 

 p. 297) was found by Schulze to be hermaphrodite : see A. N. 41 (2), p. 404. 



