VI 



PROTRACHEATA 



431 



with the Alci'ip'tdmi eye of the Chcetopoda described on p. 230. Hut the 

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

 The Pcripatus eye proceeds ontogenetically from a hollow imagination 

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

 imagination 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 2n,<i.'cf<t. 

 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 Inlidce. 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 



O 



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 FJG , 94 _ A nephridium of Peri . 



penultimate limb-bearing segment. It has, pa tus Edwardsii (after Gaffron). 



however, been proved that the ducts Of Sr, Funnel ; sj, looped canal, or ne- 



, . , , phridial duct; eft. terminal vesicle ot 



the sexual organs which emerge here the 



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, 



