GEPHTREA. CHJETIFERA. 



389 



the female of Bonellia have the same relations (fig. 313). These 

 rudimentary males are furnished (in many species) with two ventral 

 hooks, in front of which in the anterior region is placed the external 

 opening of the vas deferens. The vas deferens corresponds to the 

 uterus of the female, and is in like manner provided with an internal 

 opening into the body cavity. In Echiurus there are two pairs of 

 brown tubes, which function as generative ducts and reservoirs. 

 In Thalassema there are, according to Kowalevski, three pairs of 

 such tubes. 



The development shows many points of 

 similarity with that of the Annelida. Be- 

 tween the Achceta and Chcetifera, however, 

 there are considerable differences. In both 

 cases a metamorphosis follows the embryonic 

 development. The larvse resemble Loven's 

 larva (larva of Polygordius) ; but in the 

 Achceta they are characterised by a great de- 

 generation of the apical region (prseoral lobe) 

 and the absence of a prseoral band of cilia. 



The remarkable larva known as Actino- 

 trocha, which is the young stage of the 

 tubicolous gen vis Phoronis,* is distinguished 

 by the possession of a contractile prseoral lobe, 

 behind which there is a circle of ciliated ten- 

 tacles forming a collar. 



The Gephyrea are all marine. Some of 

 them live in sand and ooze at considerable 

 depths, also in holes in the rocks and in 

 crevices between stones and corals, and in 

 the shells of snails. Their food is similar to 

 that of Hoiothurians and many tubicolous 

 Annelids. 



Order 1. CH.ETIFERA = ECHIUROIDEA. 



FIO. 313. Planarian- like 

 male of Bonellia (after 

 Spengel). D. Intestine ; 

 WT, ciliated funnel of the 

 vas deferens (Vd), which 

 is filled with sperm. 



Gephyrea characterised by the presence of two strong hooked setce 

 on the ventral side and by a terminal anus. The mouth is placed at 

 the base of the prceoral lobe, which is developed into a jjroboscis. 



The Echiuroidea or chsetiferous Gephyrea present no external 

 segmentation of their elongated and contractile body ; they have, 

 however, in the young state the rudiments of 15 metameres. This 



* There should be a third order of Gephyrea for these animals. 



