S'ructure of 'Ma.gelona. 423 



the cuticle thinner, and there are indications, e.g. the lateral 

 frill, that the mouth is approaching. The lateral expansions 

 of the snout (which resemble alae in transverse section) also 

 diminish considerabh% their shape alters from the lanceolate 

 to a more or less cylindrical condition, and then, by a basal 

 constriction, they resume a clavate appearance. The cuticle 

 on these expansions is much thicker dorsally than ventrally, 

 and, while the chitinous raphe is placed in the lower third 

 instead of the middle of the processes, the hypotlermic fibres 

 preserve the same arrangement in front. Immediately 

 behind the transverse space alluded to above, and in a line 

 with the commencemerit of the dorsal longitudinal muscles, 

 the hypoderm somewhat increases in thickness toward the 

 base of the expansion, slightly narrows at the dorsal arch, 

 and again expands before the decided narrowing occurs over 

 the dorsal longitudinal muscles. From the inferior border 

 of the base of the lateral expansion a chitinous septum joins 

 the raphe at a somewhat acute angle, and cuts off a narrow 

 strip of hypoderm, to which and the great lobe, projecting 

 beneath the expansion, it acts as a party- wall. The ventral 

 hypoderm has now merged into the fuliaceous surface of the 

 oral region. 



The lateral expansions of the snout then • decrease (in 

 transverse section) to simple processes in which the raphe is 

 barely visible and then disappears. They spring from a 

 thick mass of hypoderm marked by certain large areolae, and 

 which gradually diminishes dorsally until the middle line is 

 reached. A slender neck of hypoderm proceeds downward 

 along the now slightly projecting process beneath the ex- 

 pansion, and then enlarges to form a bulbous ventral mass, 

 which contains the nerve-cord with its neural canal (situated 

 externally). It then becomes continuous, by a narrow strip, 

 with the buccal region. The expansion soon shrinks to a 

 blunt process, and the hypodermic band between the dorsal 

 and ventral regions elongates. The ventral hypoderm also 

 increases and its cuticular investment is thicker. At the 

 oiigin of the tentacles the hypoderm of the snout forms an 

 arch over their base, the outer edge being thick, while the 

 inner part of the arch is more translucent. In some views 

 it simulates an aperture. 



Behind the oral region the hypoderm still forms a com- 

 plete ring. A narrow arch occurs over the dorsum, but it 

 widens at each side and at intervals runs into the lateral 

 lamellae, which, with the exception of the delicate cuticular 

 investment, are almost wholly formed of this tissue. A 

 somewhat thick layer courses down the lateral region and 



