KLEINENBERG ON DEVELOPMENT OF LOPADORHYNCHUS. 539 



independently, and very little of the larval musculature passes 

 over to the adult. The larval muscles originate from two 

 ectoderm thickenings situated beneath the prototroch. Finally, 

 the larva is provided with a muscular layer, which is to be 

 found nearly everywhere between the ectoderm and endoderm. 

 The exact arrangement of the larval muscles need not detain 

 us : it only remains to be noticed that no part of the larval 

 musculature is formed from the ventral plate, whereas the 

 adult muscles are derived, if not entirely, for the most part 

 from that structure. The separation of the muscle-plate from 

 the neural plate has already been described : it must be borne 

 in mind that both owe their origin to an ectodermal cell pro- 

 liferation. The muscle-plates grow rapidly in size and im- 

 portance, partly through the addition of wandering cells 

 derived from the ectoderm. The muscle-plates become seg- 

 mented from before backwards, so that each somite has a pair 

 of muscle-plates, which are separate from one another in the 

 mid-ventral line. A seta-sac projects into the muscle-plate 

 on either side of each segment, dividing it into four regions — 

 an anterior, a posterior, a median, and a lateral region. The 

 ventral muscles of the adult are derived from the median 

 region, the dorsal muscles from the lateral region, the parapo- 

 dial muscles from the anterior and posterior regions. (See 

 Fig. 2.) In the cavities of the parapodia in the adult are found 

 peculiar stellate muscle-cells, which have no connection with 

 the ventral plate, although they appear to be remnants of the 

 larval nervous system, they are far more probably derived from 

 separate centres determined by the muscle foundations of the 

 body. 



The muscle-plate does not split into splanchnopleure and 

 somatopleure in Lopadorhynchus, nor does this happen in 

 many other Annelids (Asterope, Nauphanta, and others), but 

 the musculature of the gut is formed by the aggregation of a 

 number of single free wandering cells, derived originally from 

 the ectoderm through the intervention of the muscle-plate. 

 Thus we learn that the coelom of the adult Annelid is neither 

 au enterocoele nor a schizoccele, but is an archiccele, i.e. the 



