212 PEOF. HUXLEY ON THE 



mentary cavity is rarely formed by invagination, and cilia are 

 never met with in any part of the body*. The mesoblast is 

 developed and becomes segmented precisely in the same way. 

 Liaibs are formed and rarely remain rudimentary ; usually they 

 become jointed ; and, in almost all cases, more or fewer of those 

 which lie in the neighbourhood of the mouth are converted into 

 jaws. The perivisceral cavity is formed in the same way as in the 

 foregoing group ; so that the Arthropoda, like the Polychseta, are 

 " schizoccelous." In the higher Insecta, the embryogenetic pro- 

 cess is complicated by the development of an amnion, which 

 singularly resembles that met with in the higher Yertebrata. Mr. 

 Moseley's recently published careful examination of Peripatus 

 tends to show that this animal, formerly regarded as an Annelid, 

 is really a low and primitive form of Arthropod, and thus affords 

 evidence of the highest significance as to the relations of the An- 

 nelida with the Arthropoda. 



The true position of the Polyzoa is as yet, as I have abeady 

 said, a matter of doubt ; but the arguments of Morse, and still 

 more the recent investigation of Kowalewsky into the develop- 

 ment of the Brachiopoda, place the close affinity of the latter with 

 the Annelida in a clear light. The free larva of Argiope, for ex- 

 ample, is wonderfully similar to those of Spio and of Spirorhis ; 

 and the mantle of the Brachiopoda appears to correspond with 

 the cephalic hood of these Annelids. "When it first becomes 

 fixed, on the other hand, the young Brachiopod has jnany resem- 

 blances to Loxomma and Pedicellina among the Polyzoa. 



As regards the Mollusea properf, the larvae of the Lamelli- 

 branchiata, and of the majority of the Odontophora, have their 

 parallel in the larva of the Annelidan JPhyllodoce, while the young 

 of Dentalium and of the Pteropods correspond with the larvae of 

 other Annelids. A Mollusk appears to me to be essentially an 

 Annelid which is only dimerous, or trimerous, instead of poly- 

 mer ous. 



The development of the perivisceral cavity in the MoUuscan 

 series stands much in need of elucidation. There seems to be 

 little reason to doubt that the higher Mollusks are Schizoccelous ; 



* The like absence of ciUa is a notable peculiarity of Hirudo, among the 

 Leeches. 



t See Mr. Lankester's valuable paper " On the Development of LynwKBUsl' 

 Quarterly Journal of Microscopical Science. 



