gS MORPHOLOGY OF INVERTEBRATE TYPES 



opens is called the peristomium and carries four pairs of peris- 

 tomial tentacles. Among these the two posterior dorsal tentacles 

 are the longest and present a convenient character for the 

 identification of the sex. In males they extend as far back as 

 the ninth segment, while in females they barely reach the middle 

 of the fifth segment. 



The segments of the body do not present sufficient differences 

 for separating them into thoracic and abdominal segments as is 

 the case with many other polychaets. On the contrary, with 

 exception of the hindmost or anal segment, they are all more 

 or less alike, each provided with a pair of lateral appendages or 

 parapodia and a pair of nephridio pores or openings of the ex- 

 cretory organs at the base of the parapodia. The parapodia 

 are organs of locomotion and respiration. Each parapodium is 

 composed of two lobes, a dorsal notopodium and a ventral neuro- 

 podium. In the notopodium we distinguish a small lower ligula 

 and a large upper ligula with a dorsal cirrus. Between the two 

 ligulae is an opening through which the chetae or bristles pro- 

 trude. Their base is inclosed in the so-called chetigerous sack. 

 One bristle is much stouter than the rest and scarcely projects 

 beyond the opening. It is the aciculum. In the neuropodium we 

 find the same parts with the difference that the -ventral cirrus 

 sits at the base of the lower ligula. The parapodia of the first 

 and second segments lack the lower ligula, chetae and aciculum 

 of the notopodium, the latter being composed only of the upper 

 ligula with the dorsal cirrus. The chetae of all parapodia consist 

 of two parts: the shaft and the blade. The base of the latter sits 

 in a terminal socket of the former. The last or anal segment has 

 no parapodia. Instead it has two long anal cirri above the anus 

 which is terminal. 



Body wall and muscular system. The body wall is com- 

 posed of a cuticle and of a muscular skin layer. The former is 

 produced by the hypodermic and is perforated in many places by 

 the openings of the unicellular glands. The muscular skin layer 

 consists of circular muscles underlying the hypodermis, longi- 



