THE DEVELOPMENT OF DONDERSIA. 19 



p. l'76), in vvhirh the posterior end of the body is at first also sur- 

 rounded by the cap-like anterior part of the larva and hidden within 

 it. Since the Amphineura are very primitive representatives of the 

 Mollusean type, such comparisons are not unjustifiable, although the 

 unusual length of the body in this form alone determines a greater 

 change in the more compact larva. We shall return to this point 

 again in discussing the relationships of the Mollusca (Chap, xxxiv.) 



[Pbuvot (No. II.) has since published a preliminary account of the 

 development of Proneomenia. Unfortunately there are no illustrations 

 and, as the ontogenetic processes are very peculiar, it becomes somewhat 

 difficult to trace the origin of the definitive structures. We gather that the 

 developmental processes closely resemble those seen in Dondersia banyulensis 

 (Myzomenia), tbe earlier stages up to the formation of an invagination- 

 gastrula being practically identical in the two forms. Similarly, Proneomenia 

 develops a ciliated larva, divided into three segments and provided with a 

 long apical flagellum. The layer forming the primary invagination (?archen- 

 teron) does not correspond to the definitive entoderm, but gives rise to all the 

 tissues of the trunk. By the tangential division of its cells, it gives rise to a 

 superior entodermic mass resting upon a single layer of cells ; the latter in- 

 creases by the radial division of its cells and becomes infolded, forming three 

 invaginations ; of these tbe middle one, which remains open, becomes the 

 future proctodaeum, while the two lateral ones close and are transformed 

 into the masses of mesoderm, the lateral mesoderm-bands. The lower layer, 

 which now has the form of a vault, represents the ectoderm of the trunk. 

 The lips of the proctodaeum now grow out to form the caudal button which 

 first projects into the cavity of the ectodermal vault, but later, together with 

 the entire vault, becomes evaginated through the depression at the posterior end 

 of the larva. This conical protuberance with the caudal button and the procto- 

 daeum at its extremity represents the trunk of the young Proneomenia. The 

 entoderm still remains as a solid mass with the mesoderm-bands on either 

 side and in contact with the proctodaeum behind. The next important change 

 is the appearance of three ventral invaginations of the larval ectoderm, just 

 behind the circle of large cilia on the middle segment ; the median of these 

 invaginations, the larval stomodaeum, is merely transitory, while the two 

 lateral ones are concerned in the formation of the ectoderm and mesoderm 

 of the head. These two unite, forming a transverse band capping the anterior 

 end of the entodermic mass and prolonged posteriorly at two points to meet 

 the mesoderm-bands of the trunk ; this portion appears to form the muscles, 

 while the more dorsal elements of the invagination form the cerebral ganglia. 

 The cells of the apical plate seem to take no part in the formation of the 

 nervous system. The ectoderm of the head appears to form entirely from 

 these anterior invaginations, while that of the trunk develops from the 

 primary posterior invagination. The latter is now completely evaginated, 

 and has developed the provisional imbricated spicules. In this way the 

 young Proneomenia is developed under cover of a provisional ectoderm which 

 serves as a locomotors - organ and is thrown off at the moment of meta- 

 morphosis. The adult does not exhibit a distinct head but, during develop- 

 ment, this structure is perfectly distinct and arises quite independently of the 

 trunk. 



A very striking resemblance, possibly of great significance, is to be noted 

 between the developing Solenogastres and certain Lamellibranchia (Drew, 

 App. Lit. Lamellibranchia, No. II.). In Yolclia, we find the young Mollusc 

 developing under cover of a segmented, ciliated larval test provided with 

 an apical tuft. Here' also the trunk and head-regions originate separately, 

 the former possibly from the posterior blastoporic end, and the latter includ- 

 ing the cerebral ganglia from a pair of invaginations at the antero-ventral 



