280 J. H. ASHWOETH. 



favourably situated cells around, and the former has gra- 

 dually received a larger nerve supply. The definite sense 

 organ has resulted from a gradual conversion of a number of 

 these protected epidermal cells, perhaps at first into fusiform 

 sense cells, similar to those met with on the cirri or on the 

 general body surface of some Polychaetes. These became 

 further differentiated forming rod-like sense elements, and 

 the nervous apparatus connected with their bases became in- 

 ci'easiugly complex. As shown above (p. 274), when the 

 sense organ appears in the newly-formed segments near the 

 posterior end of Scali bregma, it is distinguishable only by 

 reason of the differentiation of very few epidermal cells into 

 rod-like sense cells, and the presence beneath them of certain 

 nerve-cells; it is never cirriform at any period of its growth. 



17. Nephridia. 



The character of the nephridia of Scalibregma is prac- 

 tically unknown. All our information regarding these organs, 

 is contained in Danielssen's account (1859), in which they are 

 described as the female reproductive organs. In his speci- 

 men, which was a large one, he found forty to forty-two 

 pairs of tubular yellowish bodies, one pair in each segment 

 of the animal "from the sixth to the anal segment." Those 

 lying in the sixth to the thirteenth were larger than the 

 others, being about four lines (8 mm.) long. These sac-like 

 bodies were ciliated internally and filled with an enormous 

 number of roundish cells, many of which contained yellowish- 

 green granules. Danielssen believed these organs to be 

 ovaries from which all the ova had been discharged into the 

 coelom, as at that time of the year (June) the coelomic fluid 

 contained an enormous number of ova. 



The nephridia are almost hidden from view by the oblique 

 muscles which are present in each chsetigerous segment 

 throughout the body (fig. 14). The nephridia of Scali- 

 bregma are not large sac- like organs as in Arenicola, but 

 slender loops, each formed by a tube bent once upon itself, 



