284 ^^^ ANIMAL KINGDOM 



live Study of adult structures yields little evidence that these phyla are 

 at all related. 



A comparison of annelid and arthropod morphology yields quite 

 different results. Although the arthropods will be described in the next 

 chapter it is convenient to indicate some of their general features here. 

 An extremely diagrammatic representation of an arthropod (Fig. 15.9 D) 

 shows many structures in common with the annelids. In both phyla the 

 body is segmented, and each segment usually has a single pair of ap- 

 pendages. Many arthropods have a long, tubular dorsal heart that is 

 more like that of the annelids than is the molluscan heart. Annelids 

 and arthropods both have a ventral chain of nerve ganglia with meta- 

 meric, lateral nerves to the body segments. Several basic differences also 

 exist, of course. Arthropods have a chitinous exoskeleton and jointed 

 appendages, their circulatory system is completely open, and the body 

 cavity is a hemocoel rather than a coelom. The similarities are such, 

 however, as to suggest a close relationship between the two phyla. 



If the early development of these three phyla is compared, it is 

 found that both the annelids and the molluscs have spiral cleavage, 

 whereas the arthropods (almost all of which have heavily yolked eggs) 

 do not. Gastrulation is similar in the annelids and molluscs, and fur- 

 ther development in many species of both phyla results in a free-swim- 

 ming larva, the trochophore (Fig. 15.10, A). Although the structure of the 

 trochophore varies considerably from species to species in both phyla, 

 no characteristic will completely separate those of the Annelida from 

 those of the Mollusca. Hence, development from the egg through the 

 trochophore is strikingly similar in these two phyla. Arthropods do not 

 have larvae of this type; all arthropod larvae, even in their youngest 

 stages, have jointed legs and other characteristics that readily identify 

 them as arthropods. 



The later development of the annelids and molluscs is quite differ- 

 ent. Molluscan trochophores develop a foot and a shell gland and be- 

 come veligers (Fig. 14.4). By further metamorphosis the veliger is 

 transformed gradually into the adult form. The general relation be- 

 tween the trochophore anatomy and that of the adult is indicated by 

 diagrams (Fig. 15.10 B) that for the sake of clarity do not indicate the 

 actual course of development for a mollusc, but do indicate general 

 body relationships. Annelid trochophores develop directly into the 

 adult form (Fig. 15.10 C). In both phyla the upper half of the trocho- 

 phore becomes only the extreme anterior end of the body, and most of 

 the adult body develops from the lower half. In both phyla the brain 

 develops by ingrowths of ectoderm from the upper half of the trocho- 

 phore, and the other ganglia develop from ventral ectoderm. Many 

 molluscs do not hatch until they have developed to the veliger stage, 

 and others hatch with the adult morphology. Trochophores occur, 

 however, in all of the classes except the Cephalopoda. Many annelids do 

 not hatch until later stages of development, and then emerge as small 

 worms. Trochophores are found only in marine annelids, the Poly- 

 chaeta and the Archiannelida. 



A comparison of later development in the annelids and arthropods 



