GENERAL FEATURES, METHODS, ETC. 19 



GENERAL FEATURES, METHODS, ETC. 



External Characters. — The Solenogastres constitute a group of marine 

 animals, which combine with features more primitive than in any other molluscs 

 numerous others indicating a high degree of modification. All are bilaterally 

 symmetrical, worm-like in form, usually nearly round in cross section, and vary 

 in shape from short thick set to very slender greatly elongated types. While 

 the average length is not far from twenty-five millimeters several species such as 

 Notomenia clavigera and Kruppomenia minima are from one to four or five 

 millimeters long, and on the other hand Proneomenia sluiteri reaches the giant 

 size of one hundred and forty-eight millimeters. 



The mouth, or more properly the atrial opening, usually in the form of an 

 elongated slit, holds an antero-ventral position, and is clearly separated from a 

 ventral median furrow, in the Neomeniina extending throughout the entire 

 length of the body. This latter structure is generally considered to be a true 

 pedal groove, the small fold included therein and abundantly provided with 

 glands being the foot. In the Chaetodermatina no external trace of these 

 organs exists, but a gap in the ventral musculature, and a thickening of the 

 muscle bands on each side of the mid line, and in Limifossor a definite pedal 

 sinus, indicate that they were present at a former time. In what must be con- 

 sidered a primitive condition the ventral furrow is posteriorly continuous with 

 what has usually been termed the cloacal cavity, which contains the openings 

 of the urinogenital apparatus, anus, and the respiratory organs. As is more 

 fully shown in a succeeding paragraph, the cloaca is in reality a mantle cavity, 

 and the two branchiae it contains in the Chaetodermatidae are undoubtedly 

 true ctenidia. On the other hand the folds of the cloacal wall, sometimes ex- 

 cessively developed and highly vascular, do not appear to be rudimentary 

 nor degenerate ctenidia. 



A cuticular sheath, often of great thickness, envelops the body, and con- 

 tains from one to seven or eight layers of calcareous spicules. Where more 

 than one layer is present groups of cells constitute papillae or organs of prob- 

 lematical use. In the adult condition all traces of a shell are absent, but in 

 the development of Myzomenia {Dondersia) banyulensis, as determined by 

 Pruvot, a stage occurs in which the embryo bears on its dorsal surface seven 

 slightly imbricating, calcareous plates. 



Internat. Anatomy. — In regard to the internal organization there are 

 numerous features that indicate a degraded condition, due probably to parasitic 



