NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 7 



derm of the coelenterates, though its cells have numerous muscle 

 processes, contains relatively few sensory and neural cells, and fibrous 

 branches of these cells are but little developed. 



The polychaete annelid larva of the trochophore type (fig. 8) has 

 a muscular system of which the elements appear to be quite analogous 

 to the ectodermal muscles of the coelenterates, though the system 

 itself is carried to a higher degree of development. Furthermore, 

 the larval muscles are parts of a neuromuscular system, since gener- 

 ally they follow the inner surfaces of nerve tracts in the ectoderm. 

 The muscle fibers are formed from cells derived directly from the 

 larval ectoderm, along with numerous small undifferentiated cells 

 that constitute a loose layer of mesenchyme distributed through the 

 haemocoele. The fibers are arranged principally in longitudinal and 

 circular tracts, though some of them extend from the body wall to 

 the alimentary canal. The endoderm of the larva does not produce 

 directly either muscular or neural cells. The nervous system of the 

 polychaete larva, when best developed, consists of longitudinal and 

 circular strands of ectodermal nerve cells and fibers following the 

 muscle tracts, and of ganglionic groups of nerve cells developed 

 particularly in connection with sensory organs on the preoral part 

 of the body. The larval elaboration of the neuromuscular system is 

 largely a temporary adaptation to the specialized form and habits of 

 the trochophore, for most of it is lost when the larva undergoes its 

 metamorphosis into the definitive worm form ; but the preoral part 

 of the larval nervous system forms the brain of the adult, and some 

 of the larval muscle fibers are taken over into the definitive muscular 

 system. 



If now we endow our hyix»thetical annelid ancestor (fig. 2 E) with 

 a primitive neuromuscular system derived from the ectoderm, and 

 provide it with a pair of primitive nephridia, it will have reached an 

 evolutionary stage entirely comparable in structure with that of 

 an annelid in the ontogenetic stage of the young polychaete larva. 

 The usual trochopore larva of the Polychaeta (fig. 4 A), however, 

 leads a purely pelagic life ; it floats upright in the water and swims 

 by means of bands of cilia that encircle the body. Its radial and 

 circular neuromuscular system appears to be entirely adapted to its 

 upright position, and many zoologists have regarded the trochophore 

 as the ancestral form of the annelids as well as of various other 

 invertebrates. The lateral position of the mouth, however, just 

 below the principal circle of cilia (Mth), gives us good reason for 

 suspecting that the shape of the trochophore and the position assumed 

 in the water are secondary adaptations to a brief swimming existence ; 



