241 



NATURAL HISTORY. 



LARVA OF GEPHYREA 



(After Gegenbaur.) 

 a, , repualip lobe, upper lip; b, lower 



but on touching the animal it contracts immediately, becomes narrow, cylindrical, shai-per at both 

 ends, and hard. Both in the uncontracted and contracted condition, the body is swollen out here 

 and there for a time and then contracted again. The Gephyrea have the sexes separate, and the 

 young undergo metamorphoses. Most of them have a superior cerebral ganglion or a double one, 

 an cesophageal nerve-collar, and a ventral gangliomc cord. This cord differs from that of the 

 a class Annelida, for although it gives off nerves from its sides there 



are no separate ganglia at regular distances ; but it is covered with a 

 layer of cells which environ a canal, and it is placed within a blood- 

 vessel. There are eye-spots directly over the brain ganglia, in some 

 genera, and the proboscis is a tactile organ. The skin is analogous to 

 that of the worms generally, but the transverse furrows do not amount 

 to segmentation ; numerous glandular follicles supplied by nerves are on 

 the skin, and open out by pores in the epidermis. Bristles are rare, and 

 there is chlorophyll in the skin of Bonellia. The muscular coats 

 beneath are stout, and the outer is circular and the inner longitudinal, 

 and the proboscis is retracted by bands o f muscles from the body-wall. 

 The proboscis is ciliated and also covered. *>vth bristles. In some the 

 pharynx is armed with teeth, and there are salivary caeca opening into 

 it. The intestinal canal is small in calibre, is within the peri visceral 

 cavity, is long, and usually coiled. It is, as is the body cavity also, ciliated 

 within. The vent may be dorsal, posterior, or even at the junction of the 



, replmlip lotic, upper lip; b, lower . 111 -JTTI i i 



lip; jc, vibratiie crown; t, intestinal proboscis and body. W hen the vascular system exists, it consists ot two 



canal; m, muscular fibres. 



long vessels, one along the median ventral line and the other dorsal, running 



along the intestine. In the genus Sipunculus these vessels are joined by one around the front 

 of the body, within, and vessels are given off from it to the tentacles. The blood may be colour- 

 less, red, blue, or violet, and there may be a similar fluid in the perivisceral cavity containing 

 amoeboid and flagellate corpuscules. 



The genus Priapulus has a branchial tuft at its hinder end, and Echirurus has branched 

 structures, receiving vessels from the ventral vascular trunk opening into the intestines. The 

 tentacles of some act as respiratory organs. Finally, there are traces of excretory or segmental 

 organs, in the form of four ciliated pouches on the lower part of the body, 

 and they vary in number and use, being sometimes in relation to the process 

 of reproduction. The males are not so numerous as the females, and differ 

 in shape. In one genus (Bonellia) the male is like a Planarian in shape, and 

 lives in the female. The eggs hatch and the embryos are free-swimming 

 and unlike the parent. Their mouth is in the front part, and is overlapped 

 behind by a double-lobed upper lip, which is round and ciliated, and on the 

 ventral side there is a small ciliated lower lip or several ciliated processes. 

 These become tentacles in some genera. Behind these lobes and mouth 

 there is a curvelet of cilia, and then follows a long bag-like body with an 

 intestine and anus. All this embryo or larva is not changed into the 

 adult, but only a part grows into the mature form. Many of the larvte 

 resemble the Rotifera in their circles of cilia, which surround the mouth, anus, 

 and body. 



The Gephyrea are divided into three orders. The Gephyrea inermia 



have no bristles, and the mouth is at the extremity of a more or less retractile proboscis. Priapulus 

 is the typical genus of the first family of this order. They are not uncommon in the Northern 

 seas. The genus Sipunculus is the most important genus of its family, the Sipunculida?, which 

 have a retractile proboscis, tentacular arms, and a twisted intestine. Phascolosoma is another genus. 



The second order is that of the Gephyrea armata, which have bristles on the anterior part of the 

 body, and posteriorly also. The genus Bonellia, already slightly noticed, belongs to it, and also the 

 genus Echirurus, which has an undivided proboscis. 



One genus (Phoronis) forms the third order of the Gephyrea tubicola. Phoronis hippocrepia 



SIPUNCULVS KEHNHARDrS. 



