788 SUMMAEY OF CURRENT RESEARCHES RELATINa TO 



B. INVERTEBRATA. 



Marine Larvae and their Relation to Adults.* — Dr. H. W. 

 Conn finds that the simplest known larva is the pilidium of the 

 Nemertines, and it seems also to be the most primitive, for it is little 

 more than a gastrula ; it has, however, one cilium or a tuft of cilia at the 

 end of the body opposite to the blastopore ; these are not locomotor 

 organs, but are carried stififly, and seem to be sensory. Around the 

 border of the bill of the larva there is, further, a special band of 

 cilia, which is locomotor in function ; both these sets of cilia are 

 accompanied by an ectodermal thickening. Although a uniform tuft 

 of cilia cannot be regarded as having any phylogenetic meaning, we 

 may surely come to some conclusions from tracts of cilia ; the bands 

 just mentioned seem to the author to indicate " that even in our early 

 pilidium larva as well as in all other larvae where this tract is repre- 

 sented, there is present a certain tract of ectodermal tissue which has 

 acquired a function different from that of the rest of the ectoderm, a 

 tract which may give rise to cilia, or sensory organs, or tentacles. 



Dr. Conn insists that the true teaching of embryology will come 

 from the union of egg embryology and larval history ; when they 

 come into conflict each case must be examined by itself ; there is no 

 general rule. 



The study of the development of TJialassema and Setyula has 

 led to the following conclusion ; " all larvae which possess in their 

 gastrula stage a circumblastoporal ring must, upon the subsequent 

 completion of the alimentary canal, have both mouth and anus on 

 the same side of this ring " ; the ring will subsequently become 

 prasoral in position. 



When elongation takes place the prgeoral lobe may be affected, or 

 the oral lobe elongates to form the body, and the praeoral remains 

 relatively very small ; the former obtains in the Coelenterata, Polyzoa, 

 and Brachiopoda ; the latter in Annelids and Molluscs, and probably 

 also in Sipunculids and Planarians. After a short account of the 

 larvae of these forms. Dr. Conn comes to the Echinodermata, with 

 which he unhesitatingly associates the Tornaria-larva of Balanoglossus. 



Here we have to do with very distinct but also very highly modified 

 larval forms ; there is nothing which is exactly a pilidium, but in 

 the corresponding stage we find a typical gastrula with a bunch of 

 long cilia at the aboral end ; there is no circumblastoporal ciliated 

 ring, and Gegenbaur "has gone beyond legitimate conclusions" in 

 homologizing their ciliated bands with those of a similar name in the 

 trochosphere. The author, in opposition to F. M. Balfour, thinks 

 the Echinoderm larva is later and not earlier than the pilidium, and 

 gives his reasons for this view. 



We may, then, conclude that all animals which possess free larval 

 forms other than Arthropods, sponges, or parasitic forms, can be 

 related to each through a form which is essentially a pilidium ; the 



* Stud. Biol. Lab. Johns-Hopkins Uuiv., iii. (1885) pp. 165-92 (2 pis.). 



