16 HISTORICAL REVIEW. 



gonad, postei-ioily becomes single, and opens into what has l>een termed the 

 pericardium, in i-eaUty an accessory part of the reproductive system; while the 

 dorsal sinus courses in a tube (hanging partially in the so-called pericardium), 

 which is called, l)y other authors, the heart. In DGndcrsia banyulensis sperma- 

 tozoa develop on the external walls of what has l^een termed the heart, while 

 the lateral walls of the pericardium are ciliated and serve to convey the sperms 

 as in many hermaphroditic gastropods. Eggs are temporarily stored in the 

 "poche accessoire" (pericardium), and the kidneys are in reality genital ducts 

 without renal fimction. 



In a later paper ('90) the same author describes a few very interesting 

 stages in the development of Dotidcrsia haui/itlensif^, and two years afterward 

 adds some further observations regarding the embryology of Proncotuenia 

 agUiopheniac. Ova in the pericardium (of other authors) lack membranes; 

 as these are present in extruded eggs it follows that the suj^posed kidneys are in 

 reality shell glands. The segmentation stages resemble those of scaphopods 

 and pelecypods, and to some extent this similarity is visible in the later develop- 

 ment. A gigantic coat of ciliated cells (a highly developed velum probably) is 

 formed, and within this the embryo forms by a process certainly not primitive 

 or at all events unlike that of other molluscs studied up to the present time. 

 When the velum is thrown off the \iw\i\ resembles to some extent a young Chiton, 

 possessing seven imbricnted calcareous plates along the dorsal surface and later- 

 alh' situated Hatteneil sjiines in what appears to be the girdle. The internal 

 organs at this stage are practically undeveloped, and as the later growth is 

 wholly unknown the present results throw but little light ujion the imjiortant 

 subject of the jihj'logenetic development of the group. 



Pelseneer ('90) considered (contra Hubrecht and otliers) that Chitonelhis 

 is not a jirimitive form intermediate between the Solenogastres and more typical 

 Chitons, Init on the other hand is highly specialized. These conclusions were 

 based on data supjilied by a study of the branchia, nervous s_ystem, ami shell. 

 In an introduction to the extensive work of Blunnich ("91) Hatschek seconds 

 Von Ihering in his attempt to place the Chitons and the Solenogastres apart 

 from the gastropods, and agrees also with Pelseneer in regard to the position of 

 Chitonelhis. 



Owing to the studies noted in the preceding paragraphs the broader fea- 

 tures of the anatomy of these animals have been settled beyond dispute, and 

 conse([uently the papers from this time forward serve in large measure to supply 

 details, and to a limited extent indicate the phylogenetic relationships of this 



