THE CTENOPHORA 



and the object is held fast by the spiral thread which remains 

 attached to the tentacle. 



According to Samassa, the lasso-cells are formed from two cell 

 elements. The hemispherical cup is the product of a meta- 

 morphosed gland cell the nucleus of which may often be dis- 

 tinguished in the convexity of the cup near the point of attachment 

 of the spiral thread. The straight, thread-like filament and the 

 spiral, contractile filament are formed from an elongate cell, which 

 is apparently a metamorphosed interstitial cell. If Samassa's 

 account is correct, it is obvious that there is no homology between 

 the lasso-cell, composed as it is of two metamorphosed cells, and 

 the nematocyst which is the entoplastic product of a single cell. 



All the Ctenophora are monoecious, the ova and spermatozoa 

 being formed from the endodermic epithelium lining the sub-costal 

 meridional canals. The ova are developed on one side, the 

 spermatozoa on the other side of each canal. In the sub-sagittal 

 canals the ova are borne on the sides nearest to the sagittal plane, in 

 the sub-transverse canals they are borne on the sides nearest to the 

 transverse plane. In Pleurobrachia and Hormiphora, as in the 

 Cydippidae generally, the ovaries and spermaries are simply 

 paired outgrowths from the walls of the meridional canals, and 

 extend as two long bands throughout the entire length of each. 

 As a rule, all the eight meridional canals bear gonads in the 

 Ctenophora, but in Euchlora rubra and CJiaristephane fugiens the 

 gonads are formed only in the four sub-transverse canals. In the 

 Lobatae and Beroidae the gonads, whilst occupying the typical 

 position, are somewhat modified in detail. In the former group 

 the meridional canals are produced laterally to form diverticula 

 underlying each comb. In Eueharis mnlticornis and Bolina alata 

 the ova and spermatozoa are found in these diverticula only, but 

 in Deiopea and Bolina hydatina the diverticula are sterile, the 

 reproductive cells being confined to the sections of the meridional 

 canals which lie between successive combs (Fig. III. 8 and 9). 

 In the Beroidae the meridional canals are produced laterally into 

 short, branched diverticula in which the sexual cells are developed 

 (Fig. X.). 



The ova in most cases are deposited singly and are fertilised 

 in the sea-water. The breeding season in Northern seas lasts 

 through the summer months, in the Mediterranean throughout the 

 year. The ovum is centrolecithal, consisting of an inner vacuolated 

 mass surrounded by a layer of granular protoplasm. It is enveloped 

 by a vitelline membrane rather widely separated from the surface of 

 the egg, the space between being filled with a gelatinous substance. 



The most interesting feature in the development of the Cteno- 

 phora is the formation of a definite mesoblast. We owe this 

 important discovery to Metschnikoff (18), whose observations 



