III. ANNELIDA: HIRUDINEI 2S1 



the latter without division into gangh'a. The relations of the Gephyra?a 

 to the ChiL-topoda are shown by the development. In some (Chatiferi) 

 there is a trochophore (fig. 267) from which the worm arises, as in the 

 Chffitopoda, by growth at the hinder end; this at first has a segmented 

 ccelom and nervous system, the metamerism being lost later. 



Order I. Chaetiferi (Armata, Echiuroidea). 



With spatulate preoral lobe, often forked at the tip; at least a pair of ventral 

 sete; a trochophore in development. Echiurus* northern, Thalassema* In 

 Bonellia there is a marked sexual dimorphism (fig. 268); female, 2 to 5 inches, 

 has a proboscis a yard long. The male, only i mm. long, totally different in 

 form and color, lives parasitically in the oesophagus of the female (fig. 268, B). 



Order II. Inermes (Achseta, Sipunculoidea). 



Distinguished by lack of cha^ta;, the mouth surrounded by tentacles, and 

 the dorsal and anterior position of the anus. The larva is a modified trocho- 

 phore without preoral ciliated band and without segmentation; only two, 

 sometimes but one, nephridia. The vascular ring around the mouth, with its 

 dorsal, heart-like prolongation, is not circulatory. It is a special part of the 

 coelom for the protrusion of the tentacles and has no connection with the in- 

 testinal blood sinus. It is doubtful whether the Inermes are related to the 

 Cha?topoda. Some unite them with Brachiopoda and Polyzoa in a group 

 Prosopygii, so called in allusion to the dorsal position of the anus. Phascolosoma* 

 (fig. 26'6). Phascolion* Siptmc'ulus.''' 



Order III. Priapuloidea. 



No tentacles, mouth with teeth, terminal anus, two protonephridia united 

 with sexual organs and opening either side of vent. Priapulus. 



Sub Class III. Hirudinei (Discophori). 



Three points of external structure distinguish the leeches from the 

 chcetopods. First, the aljsence of bristles (except in Acanthohdella) and 

 the presence of two suckers; the one on the posterior ventral surface is 

 used only for attachment and locomotion, the other, sometimes scarcely 

 differentiated, surrounds the mouth and is used in sucking the food. In 

 locomotion anterior and posterior suckers are alternately attached, the 

 body being looped up and extended like that of a 'span worm.' The 

 animals can also swim by a snake-like motion of the whole body. 



A second point is the fine ringing of the body, there being usually many 

 more rings than somites, the segments being divided by secondary con- 

 strictions, there being three, five, or even eleven rings to a segment. The 

 middle or one of the anterior rings often bears strongly developed 

 sense organs. As in earthworms, certain of the somites may develop 

 a clitellum which secretes the egg cocoons. 



