1884.] 87 [Hyatt. 



modifications, among the Batrachians for example, though adap- 

 tive, have their origin in structures found now to belong to 

 ancestral types. Balfour's remarks on the meaning of the larval 

 stages of insects (Comp. Embyol., vol. i, p. 352) are very valu- 

 able in this connection. He attributes more fully than we 

 should be disposed to do, the " origin " of larval characteris- 

 tics as due to adaptation, and also states that "on grounds 

 already indicated it may be considered certain, that the groups 

 of insects without a pupa stage and with a larva very similarly 

 organized to the adult preceded the existing holometabolic 

 groups." The presence of the collar in sponges is interesting in 

 connection with Lankester's theory (Journ. Micr. Sci., vol. xvn) 

 of a primitive architroch, based on the homology of the praeoral 

 circlet of cilia, which occurs in the larvae of Molluscs, Annelids, 

 Rotifers, and Echinoderms. " All these forms can, it appears to 

 me, be derived from a ciliated girdle which was developed in all 

 probability around the ancestral organism by a specialization of 

 the ciliated ectoderm at a time when the organism was teleosto- 

 mate." There is an apparent correspondence of this ciliated 

 band in Porifera and Echinodermata, and we can follow its devel- 

 opment in detail among Porifera, as a differentiation of the rim 

 of the blastopore, thus completing Lankester's picture of the 

 Architroch, and his explanation of the oral origin of the band. It 

 was probably primitively a mouth organ of the ancestral gastru- 

 lated Architroch, similar to the circlet of cilia in the Protozoa 

 Ciliata, and having possibly similar functions. That it must have 

 been an important organ of the probably common ancestral form 

 of the invertebrata is, according to our standard, shown by the 

 characteristics of the cinctoplanula, and its probably subsequent 

 adaptation to the purposes of attachment in the ascula is simply 

 a curious example of secondary adaptation. 



In Carneospongiae the larva has the cloacal knob and the pear- 

 form, which are parts of the ascula stage, appearing often long 

 before fixation, and the formation of the ampullae is much quick- 

 ened and concentrated. These may be formed even in the free 

 larva in some species. The resemblances to the vase-like ances- 

 tors of Ascones are thus almost obliterated, being represented 

 only in the pear-form and confined to the free larva in its last 

 period, and to the ascula stage in some forms. 



