464 ZOOLOGY sect. 



cocoon is deposited in the earth and the embryos develop into, 

 complete, though minute, worms before they make their escape. 

 The segmentation is somewhat unequal. A flattened blastula 

 (Fig. 364, A) is formed, with a large but flattened segmentation- 

 cavity. This becomes invaginated to form a cylindrical gastrula 

 (B) ; the blastopore narrows and subsequently gives rise to the 

 mouth of the adult. A pair of large mesoderm cells (m) are early 

 marked off from the other cells of the gastrula; these undergo 

 division to form a pair of mesoderm bands composed of several 

 rows of small cells which grow forwards towards the mouth. 

 By swallowing movements the embryo at this stage, having 

 burst through the enclosing vitelline membrane, takes in the 

 albuminous fluid in the interior of the cocoon, and increases rapidly 

 in size. As the embryo elongates, the mesoderm bands become 

 divided into segments, and the subsequent history of these is 

 essentially similiar to what has been already described in the case 

 of Nereis. The ectoderm is thickened on each side along 

 the line of the mesoderm bands, and the mass of ectoderm cells so 

 formed becomes arranged in a number of rows each originating be- 

 hind in a larger rounded cell or teloblast. The innermost of these 

 rows (Fig. 364, C, ner, ne. co) give rise to the ventral nerve-cord. 

 The next two rows (nph) are said by some observers to give rise 

 to the nephridia all but the funnels: but according to others the 

 nephridia, or at least all their inner glandular portions, are of 

 mesodermal derivation. The brain and oesophageal connectives are 

 formed in continuity with the rudiments of the ventral nerve-cord. 

 On the whole the development resembles that of Nereis, the 

 chief differences being such as may be traced to the non-occurrence 

 in the Earthworm of any free-swimming trochophore stage, and 

 the absence of such larval structures as the large pre-oral lobe, 

 the apical plate, the prototroch, and the larval nephridia or head- 

 kidneys. 



2. Distinctive Characters and Classification. 



The Chaetopoda are Annulata with the body made up of distinct 

 metameres, which are usually numerous and similar throughout. 

 The metameres are provided with chitinous setae developed in sacs 

 (setigerous sacs) of the epidermis, and usually elevated on muscular 

 appendages, the parapodia. There is a large ccelome divided 

 internally into chambers by transverse septa, and not in com- 

 munication with the blood-vascular system, which is nearly 

 always highly developed. The ventral nerve-cord consists 

 of a chain of ganglia. The reproductive cells are formed by a 

 proliferation of certain parts of the peritoneum or membrane lining 

 the ccelome, and usually reach the exterior through ccelomoducts 

 or through modified or unmodified nephridia. 



