CLASSIFICATION 43 



existence in the sea, and therefore independently 

 evolved by animals which are not directly related 

 to each other? Some authorities take the latter 

 view; the former is perhaps more widely ac- 

 cepted, and has even been expressed by the appli- 

 cation of the name Trochophora (Wheel-carriers), 

 as a general term for those groups in which such 

 larvae are found. These include some of the 

 higher worms, which present the typical Trocho- 

 sphere, the Brachiopoda, and the Polyzoa; while 

 variations of the Trochosphere type are shown by 

 the earliest larvae of Mollusca, the larvae of the 

 Echinoderms, and those of the Hemichordata (see 

 P- 33)> the latter bringing us, as it were, within 

 eye-shot of the Vertebrata themselves. It will be 

 seen, therefore, that the range of the Trocho- 

 sphere larva covers a large portion of the ground 

 occupied by our Grade IV. There is, however, one 

 marked exception : the Arthropoda, which seem to 

 have a prejudice against cilia in any form (since 

 they include but one animal which possess any) 

 have no example of a ciliated larva. Even their 

 simplest larval forms belong to a higher type of 

 structure, in which the shelly, jointed structure 

 characteristic of the group is already indicated. 



When we speak, however, of the occurrence 

 of the Trochosphere throughout a wide range of 

 animal life, it must be understood that its pres- 

 ence is not necessarily uniform throughout a group 

 in which it occurs. Larval forms are adaptations 

 which conform with the conditions of life for the 

 particular animal in question : and nearly related 

 kinds of animal may be without a larva. The 

 Trochosphere larva is, of course, only adapted 

 for aquatic existence, and is necessarily absent 

 in the case of terrestrial forms. 



