ANNELIDA 299 



seem to suggest that two pairs of nephridia originally be- 

 longed to each segment (Benham). The metamorphosed 

 segmental organs function in the following manner : the 

 funnel takes up the genital products out of the body cavity, 

 the nephridial ducts pass them onward, and the part dis- 

 tended into a terminal vesicle serves as a genital atrium. 

 But the terminal portion in the male apparatus of the 

 Oligochaeta may be metamorphosed into an evertible copu- 

 latory organ (thus in Stylodrilus, No. 43). 



The receptacula seminis have also been traced to nephridia, 

 of which only the ectodermal vesicular end portion is assumed 

 to develop ; but Bergh prefers to consider them metamor- 

 phosed dermal glands. They arise as tubular invaginations 

 of the epidermis into the interior of the body cavity, and are 

 surrounded by the other layers of the body- wall (Vejdovsky, 

 No. 43; Bergh, No. 5). 



Entodermal Structures. 



Intestinal Canal. — In the Polychaeta and Oligochaeta we 

 saw that the intestine arises from portions of all three germ- 

 layers. The permanent mouth is generally found at the 

 place of the blastopore, a depression of the ectoderm taking 

 place here, so that the fore-gut (just like the hind-gut, which 

 arises later) is an ectodermal structure. In those cases in 

 which the larva arises from an epibolic gastrula, and the 

 blastopore does not become the mouth, as in Rhynchelmis 

 and Psygmobranchus, the fundament of the intestine at first 

 consists of a solid entodermal mass rich in yolk (Fig. 128 A 

 and B, p. 280). The entodermal wall of the mid-gut, by 

 means of which the yolk-mass that still remains is absorbed, 

 arises by the disintegration of the more central cells, while 

 smaller cells at the periphery with less food-yolk are 

 separated from the rest of the mass and form an epithelium. 

 In this condition the intestine consists of a sac closed on 

 all sides. The fore- and hind-guts are formed by its union 

 with the ectoderm in front and behind. 



The share which the two ectodermal invaginations take in the forma- 

 tion of the fore- and hind-guts is said to be very variable in the different 



