THE ANIMAL KINGDOM. 



withdraws into it. The operculum retains both a vascular and nervous 

 supply and is probably in part respiratory 1 . 



The body-walls consist of a chitinoid cuticle, a hypodermis and 

 muscular layers, a circular and a longitudinal. The cuticle is thin in 

 Tubicola. The hypodermis consists of a single layer of cells, the outlines 

 of which are sometimes lost (?). The cells are partly supporting cells, 

 partly sensory, partly glandular. Gland-cells, the secretion of which contains 

 rod-like bodies, as in Nemertea, are very common, especially in tubicolous 

 families, e. g. Chaetopteridae, but are by no means restricted to them. The 

 rod-like bodies have been found in the walls of the tube of Sabella. 

 Protective tubes are manufactured by some Errantia and all Tubicola. 

 In the Errantia and some Tubicola, e. g. Pectinaria> the tube is carried 

 about by the animal ; in other Tubicola it is formed in sand or mud or 

 is attached to stones. It may be soft in consistency, e. g. Siphonostoma ; 

 tough and parchment-like, e. g. Chaetopterus ; or stony, as in Serpulidae. 

 It is often strengthened by particles of mud, sand, or shell, e. g. Sabella, 

 Hermella, Terebella. The material for the tube is probably secreted by 

 the glands of the body-surface 2 . Cilia are found on the prostomium of 

 the Oligochaete Aeolosoma and in Polychaeta chiefly on the sides of the 

 prostomium or somites, round the anus, or on the branchiae. Ophyotrocha 

 puerilis (Eunicidae) retains the polytrochal rings of cilia (infra, p. 606). The 

 Sabellinae possess a ventral ciliated furrow in the abdominal region, which 

 either ends at the thorax, or passing round its right side, extends along its 

 dorsal median line. The ventral surface, rarely the dorsal, of Serpulinae 

 is ciliated but not furrowed. The cilia in both instances convey faecal 

 particles forwards and out of the tube. The circular layer of muscles may 

 be absent, e. g. in Nephthys, or be sparingly developed. The longitudinal 

 layer is commonly disposed in Polychaeta in four bands, two latero-dorsal, 

 two latero-ventral. The internal ends of the setiparous sacs penetrate and 

 therefore divide the corresponding layer in Oligochaeta, The setiparous 

 sacs have special protrusor muscles, derived in Oligochaeta from the circular 

 layer. Muscular bands pass from the median ventral line in Polychaeta, in 

 an oblique dorso-ventral direction and are inserted into the dorsum, the 

 parapodia, and the setiparous sacs. They consequently divide the coelome 



1 In Filogj-ana and Apomatus the peduncle of the operculum bears lateral processes, and the 

 stopper is represented by a terminal sphere. For the metamorphosis of the filament, cf. F. Miiller, 

 Facts for Darwin, 1869, p. 112. 



2 Claparede thought that the material for the tube was secreted entirely by the most anterior 

 pair of nephridia, the only pair, as he supposed, present in Serpulidae. Pruvot observed the posterior 

 part of a Myxicola, accidentally divided, envelope itself in a tube of mucus, undoubtedly derived in 

 this instance from the hypodermic glands. The presence of the characteristic rods alike in the gland 

 cells and in the substance of the tube of Sabella points to the same conclusion. It is also the natural 

 conclusion judging from the analogy of Nemertea. Aggregations of unicellular glands open on the 

 ventral surface of certain segments in Polydora, and, according to Jacobi, secrete material for the 

 tube. 



