THE POLYCH^TA 49 



on its ventral side. The buccal cavity and pharynx are often 

 protrusible and provided with powerful chitinous jaws and 

 plates. The body is segmented, but the internal septa often 

 break down so that there is a continuous ccelomic space 

 surrounding the gut. The gonads are developed from the 

 ccelomic epithelium, and instead of being restricted in position 

 as in the oligochaeta, are more scattered over the body, 

 occurring in the cavities of the parapodia, on the course of 

 the oblique muscles which traverse the ccelom, on the course 

 of the blood-vessels, on the walls of the intestine, or even on 

 the body-wall. Nephridia generally occur in all the segments 

 except a few of the most anterior and posterior, but are reduced 

 in number and restricted to certain segments in some forms. 



The parapodia are hollow lateral processes of the body.- 

 Usually there are no parapodia on the mouth segment 

 (peristomium), but they are present on all the other segments 

 except the last. The parapodia are often double, consisting 

 of a dorsal branch called the notopodium, and a ventral branch 

 called the neuropodium. Each bears a bunch of chaetae, among 

 which specially large deeply embedded ones are known as 

 acicula. Typically both notopodium and neuropodium bear 

 filiform appendages called cirrhi; these may be modified in 

 various ways to form branchiae, or flattened protective scales 

 called elytra. Many of the Polychaeta pass through a trocho- 

 sphere stage in development. The trochosphere may be more 

 or less disguised, but in some cases it presents a remarkably 

 close resemblance to the larva of Polygordius. 



The Hirudinea or leeches are elongated vermiform animals 

 with an external annulation which does not correspond with 

 the internal segmentation. One member of the class has 

 chaetae, but these structures are absent in all other leeches. 

 The ccelom is curiously broken up into a number of spaces 

 and channels, and the remainder of the body-cavity is filled 

 with a peculiar form of tissue known as botryoidal tissue. A 

 description of the medicinal leech is given in Marshall and 

 Hurst's " Elementary Zoology," from which the student can 

 learn all the important facts concerning its anatomy. 



The Echiuroidea comprise a small group of peculiar vermi- 

 form marine animals provided with a pair of hooked chaetae. 

 In the case of two members of the group there is a typical 

 trochosphere larva. 



II. D 



