256 MANUAL OF ZOOLOGY. 



organs resembling the " respiratory tree " of the Holothurians 

 (as in Echiurus\ the latter, however, being rather excretory 

 than respiratory in function. The nervous system has the form 

 of a gangliated cesophageal ring, giving off a ventral cord, 

 which is peculiar in not having distinct ganglia developed upon 

 it. The sexes are in different individuals (united in some 

 species of Sipuncnlus] ; and the males may differ greatly from 

 the females in form. The reproductive elements often reach 

 the exterior by the ducts of certain ciliated sacs, which open 

 by pores on the ventral surface of the body, and which corre- 

 spond with the " segmental organs " of the Annelides. 



In their development, the Gephyrea pass through a metamor- 

 phosis, generally of a very striking character. The larva (fig. 

 125) is locomotive, and swims about actively, partly by means 

 of a ciliary ring placed round the neck, 

 and partly by means of ciliated lobes de- 

 veloped on the head. In this "actino- 

 trocha" stage the larva resembles that of 

 the Echinoderms in many respects ; whilst 

 the ciliated cephalic lobes call to mind the 

 trochal discs of the Rotifers. The further 

 process of development is also strikingly 

 similar to that characteristic of the Echino- 

 derms, a considerable portion of the larva, 

 including the ciliated cephalic expansion, 

 being ultimately absorbed. 



The Gephyrea are exclusively inhabitants 

 f ^e sea, mostly burrowing in the sand, 

 ceeded to some extent, or hiding in crevices in the rocks. The 

 principal and most widely distributed genus 

 is Sipunculus, the species of which often inhabit the cast-away 

 shells of Univalves, and which ranges from the littoral zone down 

 to a depth of 2500 fathoms. 



As to their affinities, the Gephyrea are related, on the one 

 hand, to the Holothurians, from which they differ in the absence 

 of an ambulacral system,* in the fact that the integument is 

 almost universally incapable of secreting calcareous matter, 

 and in the absence of any traces of a radiate arrangement of 

 the nervous system. On the other hand, they show their close 

 relation to the true Annelides, by their general possession of a 

 pseudohaemal system, and of " segmental organs," by the form 

 of the nervous system, and by the resemblance of their larvae 

 to those of many of the Errant Ringed Worms. 



* Sipunctilus has a system of peculiar water-vessels developed in con- 

 nection with the oral tentacles, which may, perhaps, be homologous with 

 the ambulacral vessels of the Echinoderms. 



