TYPE ANNELIDA. 213 



as Serpula and Sabella, plume-like branchiae supported by an 

 axial cartilage-like skeleton occur upon the sides of the head, 

 and numerous eyes may be found in the hypodermis of these 

 structures. Parapodia are as a rule but slightly developed, 

 sometimes being entirely wanting though the setae persist, 

 those of the lower parapodial lobe being usually hook-like. 

 The protrusible pharynx with chitinous teeth does not for the 

 most part occur. Ampliitrite (Fig. 101) lives in tubes in sand, 

 while Terebella composes tubes by gluing together particles 

 of sand. In Sabella the tubes are membranous in character, 

 while Serpula manufactures more or less contorted tubes of 

 carbonate of lime. 



Development of the Polychceta. — An important feature in 

 the development of the Polychseta is the occurrence of the 

 Trochophore larva. A typical example of this larva is to be 

 found in the development of Polygordius ; it is a transparent 

 organism, having the form of two low cones united by their 

 bases (Kg. 102). Just below the junction of the two cones is 

 the mouth (if), leading by a short stomodceum or oesophagus 

 into a retort-shaped stomach, the intestine opening at the 

 apex of the lower cone. Above the mouth, along the line 

 where the two cones are united, lies a band of strong cilia 

 arranged in two rows and forming an almost complete girdle 

 for the body, being wanting, however, in the mid-dorsal region. 

 This is the prototroch {pro) or praeoral band of cilia, and par- 

 allel to it is a second weaker band which passes behind the 

 mouth — the paratroch {po) or postoral band. The slight 

 groove between the two bands is lined by fine cilia, the adoral 

 cilia, and in some Trochophores a band of fine cilia extends 

 backwards along the ventral surface of the body towards the 

 apex of the lower cone. 



At the apex of the upper cone is a strong thickening of 

 the ectoderm, the apical plate (ap), which is nervous in function 

 and bears a number of strong cilia and may also have imbedded 

 in it pigment-spots which function as light-percipient organs. 

 From the apical thickening four nerve-cords (m) extend back- 

 wards, one being dorsal, the other ventral, and the remaining 

 two, stronger than the others, lateral. A series of fine nerve- 

 rings arranged concentrically about the apical thickening unite 



