336 POLYCHAETA 



CHAP. 



small rounded knob (prostomium), which in S. spinosa Sluiter, 

 is prolonged outwards on each side to form a grooved palp- 

 like organ, recalling that of Bonellia. The anus is placed 

 posteriorly ; and in front of it, on the dorsal surface, are two 

 bundles of many long thread-like gills. On the ventro-lateral 

 surface, in front of the middle of the body, is a pair of finger- 

 shaped processes containing the genital ducts. The anterior 

 segments of the body can be withdrawn into the body, as in the 

 Chlorhaemidae. Further examination leads to the conclusion 

 that the body of Sternaspis consists of about thirty segments, 

 most of them provided with paired bundles of capillary (neuro- 

 podial ?) chaetae, distributed as follows : Each of the segments 

 2, 3, 4 has a half ring of strong chaetae on each side ; segments 

 5 to 7 are without chaetae ; segments 8 to 14 have chaetae em- 

 bedded in the body-wall, but not projecting. The shields cover 

 the remaining segments ; and along the outer edge of each are some 

 ten bundles of chaetae, and along the hinder edge some five or six 

 bundles, representing as many segments. Thus the worm consists 

 of about thirty segments whose outlines are nearly obliterated 

 (as in the Chlorhaemidae), and whose chaetae, except those which 

 are specially developed, are ' disappearing : while posteriorly a 

 great shortening of the body brings the bundles close together. 

 A continuation of this process, involving a further disappearance 

 of chaetae, leads readily to the condition met with in Echiurus, 

 one of the chaetigerous Gephyrea. Internally, further evidence 

 of the relation between Sternaspis and the Chlorhaemids with 

 the Gephyrea is afforded by the absence of septa, by the coiled 

 alimentary canal, and by the presence of a single pair of 

 nephridia, which in the latter group act both as excretory 

 organs and as genital ducts. t 



BKANCH B. CBYPTOCEPHALA. 



Sub-Order 1. Sabelliformia. 1 



Fam. 1. Sabellidae. The branchial crown consists of a 

 usually considerable number of filaments arising from a semi- 

 circular base. The peristomium may be reflexed to form a collar, 



1 For anatomy see Meyer, 3ft. Zool. Stat. Neapel, vii. 1887. 



