Strucdo'e of ^lagi^hnn. 421 



cuticle, as in this case, often coinciding- with entire absence 

 ofciha). Instead ot that disintegration of surface which so 

 readily takes phice in the latter, it is often .found that the 

 tough hyaline cuticle holds together, as in a bag, the decaying 

 and pulpy internal tissues. In this form the cuticle is 

 characterized by its great strength on the head and anterior 

 region of the body, so as to enable it to resist the friction 

 inseparable from such active habits in the sand. M. Clapa- 

 rede observes that the sedentary annelids are characterized 

 by the delicacy of this coat, a condition he attributes to the 

 special protection afforded by their tubes; and the habits of 

 Mageloiia would explain the structural ditierences in this 

 respect between the anterior and posterior regions of the 

 body. 



The habits of Magelona seem to be too active for the 

 attachment of external parasites (Infusoria &c.) which are 

 common in Nephthys and other forms that burrow in the 

 sand. 



2. The Hypoderm. 



The hypoderm forms a vgry large proportion of the tissues 

 of the flattened snout *, the whole region outside the four 

 muscular compartments being occupied by it; and when the 

 snout attains full development the lateral hypodermic expan- 

 sion almost equals in breadth the entire muscular region. 

 The lateral expansion is often deeply frilled during life, and, 

 moreover, presents certain secondary folds within the edge — 

 features which would lead one to suppose that it contains no 

 muscular fibres. In front of the dilatation at the origin of 

 each tentacle a semilunar fold, with the concavity directed 

 outward, occurs at the margin. The hypoderm also forms a 

 wedge-shaped mass between the median muscular compart- 

 ments anteriorly. After the muscular chambers are fully 

 formed the snout may be divided into a median and two 

 lateral regions, though, of course, the tissue is quite con- 

 tinuous. 'J'he dorsal hypoderm forms an almost uniform 

 belt of cells and granules, which, in vertical transverse 

 section, present numerous nearly vertical streaks. The 

 breadth of the ventral hypoderm is greatest toward the 

 median line, diminishes at each side to a mere streak under 

 the external muscle, and again expands in the lateral region. 

 The symmetrical streaks of the median part (which is more 



* Ranzfinia, a form inchided under the Cbaetopteridse by M. Claparede, 

 has a large bilobed flattened snout (Supplement, ' Les Annel. Chetopodes 

 du Golfe de Naples,' p. 120, pi. xi. fig. 1). 



