206 THE ANATOMY OF INVERTEBRATED ANIMALS. 



an appendage of such a nature as the labrum or the rostrum 

 of a Crustacean, there is no evidence at present to show. 



It is highly interesting to remark that thus, in the Poly- 

 nbe, as in the Arthropoda, the "head " results from the modi- 

 fication of a number of somites, some of which lie in front of, 

 and others behind, the mouth. The movements and evident 

 extreme sensitiveness of the inferior prsestomial cirri during 

 life indicate that they perform the functions, as well as occupy 

 the position, of antennas. 



The hindermost segment of the body, or pygidium (Fig. 

 51, JB, Fig. 53, A), is narrow, and divided at the end into two 

 supports for the pygidial (d) cirri which are as long as the 

 three last somites, and resemble the notopodial cirri in form 

 and structure. They extend directly backward, almost paral- 

 lel with one another, and with the notopodial cirri of the last 

 somite, w T hich are thrown backward and downward (Fig. 53, 

 A, c). It seems probable that the pygidium represents only 

 a single somite. 



The anus is not terminal, as in many Annelids, but is 

 seated in the middle of a strongly-raised papilla (Fig. 53, 

 A, x), which projects from the dorsal surface of the penulti- 

 mate somite ; its sides are produced into about fourteen folds. 

 The two last elytra have their edges excavated, so as to leave 

 a space over the anus (Fig. 51, A,f). 



The notopodial cirri and the elytra do not coexist upon 

 the same somites ; and the order of arrangement of the ely- 

 trigerous and cirrigerous somites is very curious. The 1st or 

 peristomial somite is cerrigerous, and so are the 3d, 6th, 8th, 

 10th, 12th, 14th, 16th, 18th, 20th, 22d, 24th, 25th, and 26th ; 

 while the 2d, 4th, 5th, 7th, 9th, 11th, 13th, 15th, 17th, 19th, 

 21st, and 23d, somites bear elytra, making twelve pairs in all. 



In no polychaBtous Annelid is the structure of a somite 

 more complex than in Polynbe ; and there are but very few 

 parts not found in Polynbe to be met with in other Annelida. 

 The careful study of this species, therefore, furnishes us with 

 an almost complete nomenclature for the external organs of 

 the whole group ; and it will be found that the other forms 

 of Annelida differ mainly in the greater or less development 

 and modification of the organs which have just been de- 

 scribed. A large proportion of the Polychceta are like Poly- 

 nbe, free and actively locomotive animals, which rarely fabri- 

 cate tubular habitations, and are therefore termed Errantia ; 

 they possess a praestomium, usually provided with eyes and 

 feelers, and have many parapodia, which are not confined to 



