GEPHYREA. 623 



and testes.' In the Echiuridae the glands are formed at the posterior end 

 of the ventral vessel, and from the coelomic cells which constitute its walls. 

 In Thalassema and Echiurus the young ova and corresponding male cells 

 are simply enlarged superficial coelomic cells, which are set free and 

 develope into ova and sperm. In Bonellia each cell-mass thus liberated, 

 consists of an envelope of delicate coelomic epithelium inclosing a mass 

 of cells, one of which is larger and becomes ovum, whilst the rest supply 

 it with secondary yolk. The Gephyrean egg is inclosed in a membrane 

 which is generally striated. 



The dimorphic male of Bonellia viridis has the following structure. 

 It is a Planarian-like organism, 1-5 mm. long, pointed at each end. The 

 hypodermis consists of a layer of ciliated cells with a delicate cuticle. The 

 muscular layers of the body-wall are an outer circular, a spiral and an 

 internal longitudinal layer (Spengel), with a mass of vacuolated cells lodged 

 in a cell reticulum, and covered internally by flat coelomic epithelium, 

 which is reflected over the intestine and vesicula seminalis. Dorso-ventral 

 bundles of fibres placed at regular intervals and projecting into the 

 coelome, give an appearance of septa. The coelome does not extend up to 

 each end of the body. The nervous system consists of a peri-intestinal 

 ring and ventral cord. The intestine ends blindly in front and behind, 

 and is attached at both ends by muscle-fibres to the body-walls. Its own 

 walls consist of a single layer of cells ; its contents are fatty. Two simple 

 excretory organs, the homologues of the anal caeca of the female, are 

 situated posteriorly, one on either side, in front of the end of the intestine. 

 Each of them consists of a tube with sparsely ciliated walls hanging in the 

 coelome, into which it opens by a ciliated funnel. Its external pore is 

 minute. The spermatozoa are formed from peritoneal cells, and are diffe- 

 rentiated from an outer layer of cells surrounding a central cell, the latter 

 undergoing atrophy. The vesicula seminalis opens at the anterior apex of 

 the body. Its duct is narrow, and passes inwards through the nerve-ring, 

 when it enlarges into an elongated sac. The sac opens into the coelome by 

 a ciliated funnel, which Spengel believes communicates with the sac itself, 

 not terminally but laterally. The male larva clings to the under side of 

 the prostomium of the female, and migrates thence into the mouth, where 

 it undergoes its final changes. It then wanders into the outer chamber of 

 the uterine pouch, which in B. minor at least is cut off by a fold from the 

 inner portion where the ova accumulate. The males of B. viridis, and of 

 Hamingia, are found in the same place. They are furnished with a pair 

 of chitinoid hooks, placed anteriorly and ventrally *. 



The larva of Echiurus is a typical Trochosphere, The most important features 



1 Spengel's male Bonellia with a pair of ventral hooks probably belonged to B. minor. Cf. p. 

 41 1 of his paper, cited below. 



