VI 



PROTRACHEATA 



431 



with the Aldopidan eye of the Chcetopoda described on p. 230. But the 

 space containing fluid between the lens and the rod layer is wanting. 

 The Peripatw eye proceeds ontogenetically from a hollow invagination 

 of the cephalic ectoderm near the rudiments of the brain. The 

 invagination closes and becomes the optic vesicle. The connection 

 with the brain is said to arise later by the growing out of the optic 

 nerve from the brain. 



The circulatory system consists of a contractile dorsal vessel or 

 heart running through the body from the first segment to the last but 

 one. This heart is supplied with paired ostia arranged segmentally 

 and provided with valves. It lies in a pericardial sinus imbedded on 

 its ventral side in a tissue comparable with the fat body of the Insecta. 

 This sinus is incompletely separated from the underlying body cavity 

 by a horizontal septum. The septum, which is formed of endothelium 

 and muscle fibres running transversely, is fenestrated on both sides of 

 the middle line. A median longitudinal nerve runs on the dorsal wall 

 of the heart, as in the lulidce. Besides the heart a very fine medio- 

 ventral longitudinal vessel is said to occur. 



Excretory organs (nephridia). Each trunk segment of Peri- 

 patus is provided with a pair of nephridia. The nephridia lie in 

 the lateral divisions of the body cavity and 

 emerge on the under side of the extremities 

 near their bases. Each nephridium (Fig. 

 294) consists of the 3 following parts : 

 (1) a terminal vesicle which opens out- 

 wards through the external aperture ; (2) 

 a looped nephridial canal bent back upon 

 itself, which ends in (3) a funnel opening 

 into the body cavity and placed near 

 the terminal vesicle. We here find then 

 the typical structure and arrangement 

 of an Annulatan nephridium. In the 4th 

 and 5th segment the nephridial canal is 

 distinctly longer and forms many loops. 

 The nephridia of the first three trunk* seg- 

 ments are much smaller than the others ; 

 their canal is short and without loops. 

 According to the species, the nephridia 

 are apparently wanting in the penultimate 

 or antepenultimate segment, i.e. the last or 

 penultimate limb-bearing segment. It has, 

 however, been proved that the duets of 

 the sexual organs which emerge here 

 are transformed nephridia. Transformed 

 nephridia are also found in two so-called anal glands, which 

 open in the last body segment (the anal segment without extremi- 

 ties) near the anus. These glands are wanting in the adult female, 



FIG. 294. A nephridium of Peri- 

 patus Edwardsii (after Gaffron). 

 tr, Funnel ; sg, looped canal, or ne- 

 phridial duct ; eb, terminal vesicle of 

 the nephridium. 



