40 Dr. C. Collingwood on the Nudihranchiate Mollusca. 



vegetable kingdoms, of the fallacious and variable character of 

 colour, and its untenability as a specific distinction. 



The development of the Nudihranchiate Mollusca has not 

 been clearly traced through all its stages ; but the hatching of 

 the ova may be readily observed ; and from these ova there are 

 produced little shelled Mollusks, or Nautilines, which are 

 freely moveable by the aid of large ciliated lobes ; but whether 

 they undergo further metamorphosis has not been satisfactorily 

 ascertained. There can be no doubt, however, that in order 

 to comprehend with correctness the affinities of new species, 

 some knowledge of the immature forms of ascertained species is 

 necessary. The relative degree of development of certain organs 

 can hardly be arrived at by a-pri07'i reasoning ; and an examina- 

 tion of the animals themselves in various stages of growth is 

 requisite for the formation of correct conclusions. Thus, in the 

 individual which has been the cause of these remarks, although 

 the general conformation of the body presented evident signs of 

 its not having arrived at maturity, the dorsal tentacles were 

 well developed, and of a much larger relative size than in the 

 mature Triopa. The tongue also, which is an organ of the 

 utmost importance in determining species, appears to have been 

 in this specimen well developed ; and a careful examination of it 

 with high powers of the microscope revealed the curved lateral 

 denticles which are described as belonging to Triopa claviger. 

 Whenever a doubt arises as to whether an animal is a new 

 species or the young of a known species, the tongue is a useful 

 criterion which may be safely depended on, and the more valu- 

 able because it is one of the hardest and most imperishable 

 structures of the Nudibranch, and may serve to determine after 

 death, a species which we have failed to recognize during 

 life. The spicula also are important hard structures, though 

 more variable than the tongue. It is remarkable that in my 

 specimen — immature, be it remarked — the spicula were more 

 highly developed than usual, being for the most part cruciform, 

 while those of Triopa are usually triradiate. Mr. Alder remarks 

 upon this, " I have drawings of the spicula of Triopa claviger 

 very similar to yours, cruciform or dagger-shaped ; but the tri- 

 radiate are the more common kinds." 



The distribution of these spicula in the Nudihranchiate order 

 is worthy of attention. They are usually calcareous and situated 

 in the cutis. The Dorididse invariably possess them in large 

 quantities, especially the Doridinse or true Dorids, in which they 

 are more abundant and are arranged more symmetrically than in 

 the Polycerinse. The Tritoniadse do not possess them, and the 

 Eolididse also are entirely devoid of them, the skin in these last 

 being soft and pliable, and the cloak absent. I once believed I 



