464 THE ANNELIDA POLYCHAETA. 



teen somites, and a longer posterior region. The thorax has a cuticular mosaic, 

 and its parapodia are rudimentary and bear capillary setae. The abdomen is 

 armed with rows of crochets. The dividing plane between thorax and abdomen 

 is at the point of transition between oesophagus and stomach. The color is 

 yellow or brown, with the anterior region nearly always blood-red. Occasionally 

 the posterior region is nearly transparent. 



The prostomium is pointed, more or less conical, retractile. It lacks tenta- 

 cles and palpi, but bears a pair of large retractile nut-formed ciUated organs. 

 Eyes have the form of pigment specks. 



The peristomium, or first somite, always lacks setae and appendages. Some- 

 times, however, it is fused with the prostomium (Capitella) and in such case 

 the first distinct somite is setigerous. 



The thorax as a whole is widest near the middle, thus more or less fusiform. 



The parapodia are uniramous. Those of the thorax are shght prominences 

 which lack all processes, and are retractile. Those of the abdomen are in the 

 form of slightly retractile, not prominent, tori. 



Branchiae are usually present in the posterior region at the ends of the rows 

 of crochets. They are commonly markedly retractile, either completely so, in 

 which case they are branched, or incompletely retractile and simple. They 

 contain coelomic fluid, or haemolymph, and hence are red from the corpuscles 

 in the latter. 



On each somite there is a pair of prominences, the so-called lateral organs, 

 covered with sensory hairs and regarded as auditory in function. Each is 

 situated on the lateral line in a groove over the thorax, but is freely exposed on 

 the abdomen. These lateral organs are lacking only in Capitella. 



There are no blood-vessels, the blood being mixed with the lymph as 

 haemolymph in the many branched coelom. The blood corpuscles are red; 

 the plasma colorless. 



The nephridia may occur in most somites of the body or may be confined 

 to a restricted region. 



The reproductive organs open through special pores, in the male showing 

 a differentiation of vesiculae seminales and penes, and in the female of oviducts, 

 receptaculae seminis and vulvae. In nearly all forms copulation occurs. Cer- 

 tain genera possess copulatory organs formed by specially modified, stout spines, 

 or hooks, of certain parapodia, these being present in the male only, or in other 

 cases in both sexes. Rarely there is no genital pore (Eisigella, Clistomastus) . 



Mouth ventral in position. Proboscis eversible, papillose but wholly 



