ON HAPLOBRANCHUS. 175 



The branchiae have become modified owing to the great de- 

 velopment of the secondary filaments, which arise alternately 

 and attain the same length as the main stems. 



In Haplobranchus, prostomium and tentacles closely 

 resemble those of Amphiglena, but the palps are well devel- 

 oped. There is some little uncertainty about the determina- 

 tion of the organs I have so marked as palps, but their 

 slightly greater muscular development, their blood supply, and 

 their close connection with the ventral region of the prostomium, 

 point to their being different in nature to the other tentacles. 



The branchial tentacles it is which are so especially inter- 

 esting in Haplobranchus, their definite arrangement, united 

 in pairs at the base, the absence of any secondary filaments, 

 the rich ciliation upon their inner faces,^ and the absence of 

 any branch of the closed vascular system in this lumen are all 

 interesting characters, and point to their being similar to the 

 peristomial tentacles of other annelids which are just taking 

 on that branchial function which is the most marked feature 

 of the whole family. These tentacles on account of their 

 united bases may really be said to represent two parapodia on 

 each side, each possessing a notopodial and a neuropodial 

 ramus, indeed, they remind one very forcibly of the peristomial 

 tentacles of such an annelid as Nereis. Their condition 

 seems to me very strong evidence in favour of their peristomial 

 nature, and consequently of the peristomial nature of the 

 branchiae of the Serpulidae. This view was entertained by 

 Milne Edwards,^ but De Quatrefages ^ states that the branchiae 

 of the Serpulidae receive their nerve supply from the supra- 

 oesophageal nerve ganglion, and consequently he considers 

 them to be prostomial. 



Claparede and Mecnikow ^ have shown that in Dasychone 



' Since sending my drawings to the press, I have observed a distinct ten- 

 dency to a grouping in tlie arrangement of the cilia ; upon the surface of the 

 branchiae, groups of cilia springing from a very slightly raised serpentine ridge. 



2 ' Kegne An. ill.,' pi. 1 E, explanation of fig. 2. 



3 De Quatrefages, ' Hist. Nat. des Ann.,' tome ii, p. 401. 

 * ' Zeit. fiir wiss. Zoologie,' Bd. xix, 1869, Taf. xvi. 



