ANATOMY AND CLASSIFICATION OP THE ARENICOLID^E. 465 



ones heaped together as in A. marina. In addition to these 

 there are colourless vacuolated and granular cells. 



In older specimens (200 mm. long) the number of ingrowths 

 from the posterior wall of the heart has increased. A few 

 are also developed from the opposite wall, and in these the 

 neck of the involution is hollow, showing the nature of the 

 ingrowth, involving, as it does, the entire thickness of the 

 wall of the heart, and a virtual extension of the coelom into 

 the processes (PL 25, fig. 42). 



In A. ecaudata the appearance of the heart-body is very 

 similar to that of A. Grubii. It is not present in post- 

 larvas (7*2 and 9*5 mm. long), but in worms 120 and 150 mm. 

 long it is well developed. In places the outer wall appears 

 to consist of an endothelium, then follows the muscular layer, 

 and finally the peritoneal cubical cells, which form an irre- 

 gular outer layer, within which are scattered granular and 

 glandular cells, the former Avith the usual yellow chloro- 

 gogenous bodies. 



The suggestion, first made by Eisig (1887), as to the 

 nature of the heart-body, and lately confirmed for Cirratulidae 

 by Picton (1898), namely, that this body is a modified 

 portion of the peritoneal tissue, receives further support 

 from these observations on Arenicola, but it is necessary 

 to discuss this conclusion more fully. 



In Terebellids and Cirratulids, Picton (1898) came to the 

 following results. The heart-body is developed by an invo- 

 lution of the anterior end of the dorsal vessel on its under 

 surface, and first makes its appearance in Terebellid larvae 

 1"5 cm. in length. Probably by proliferation of the ingrowth 

 the body increases in extent, and a secondai'y lumen appears 

 in its later strands. In the adult " the organ is enclosed in 

 an endothelium, within which cortex and medulla, seen, e. g., 

 in Audouinia, are not distinguishable, but the elongated 

 cells of which the structure is composed form a sort of 

 network, radiating out from the central axis. There is no 

 great lumen, but numerous intercellular spaces appear. The 

 pigment granules are of a greenish-yellow colour, and each 



