148 



COELEXTERATA. 



is at the top of the stem, the youngest nectocalyx bud next to it, while the 

 oldest nectocalyx is at the lowest end of the siphosome, i.e., next the youngest 

 connidium bud. 



The development of the egg leads to the formation of a variety of the 

 Siphonula larva (Fig. 123). It is a medusoid composed of a nectocalyx (A) 

 the umbrella, of a cylindrical mouthless process the manubrium, and of a 

 tentacle. The mouthless process is attached to the ex-umbrella surface of 

 the nectocalyx (to what is called in Siphonophorau parlance its ventral side), 



and is regarded by 

 Haeckel as the original 

 siphon. It is supposed 

 to have protruded 

 through a fissure in the 

 ventral wall of the nec- 

 tocalyx. This disloca- 

 tion* of the siphon (if 

 it really exists) from its 

 proper position in the 

 nectocalyx is an example 

 of a widespread phenom- 

 enon in the Siphono- 

 phora, which accounts 

 for a good many of the 

 peculiar features of the 

 group. 



The alternation of 

 generations in this 

 order is between the 

 polygastric colony and 

 the monogastric cormi- 

 dium, produced by bud- 

 ding from the former 

 and when detached 

 known as a eudoxid. f 

 The gouophore of the 

 eudoxid after shedding 

 its genital cells is cast 

 off and a new gonophore 

 is formed. 



Fain. 1. Eudoxidae. 

 Monogastric, connidium 

 composed of two rnedu- 

 soids, a sterile and a 



en- 



s.u. 



- -p. 



"I\\\\W" 



FIG. 123. Developing Siphonula larva of Mmjijhirn Kocli.ii 

 with buds on the ventral surface. A, rudiment of primary 

 neetocalyx, with somatocyst of, and commencing jelly ga 

 between ectoderm el' and endoderm en,; su sub-umbrella; 

 t budding tentacle; p siphon; en' yolky endoderm cells 

 which are absorbed later (after Chun). 



* This attachment of the primary siphon to the ex-umbrella surface of the 

 primary nectocalyx is a serious difficulty to the medusome theory. The 

 difficulty may be got over by supposing that the primary siphon is the 

 manubrium of an umbrella which has disappeared, and that the primary 

 nectocalyx is the first bud from the persistent manubrium. 



t In some forms the primary gonophore loses its sexual manubrium, and 

 is developed into a special nectocalyx, and a secondary gonophore is formed. 

 In this case the cormidium is composed of three medusoids, and is called an 

 Ersaeid. 



