8IPUNCUL0IDEA. 



By W. F. Lanchester, M.A., 

 King's College, Cambridge. 



Pha.scolosoma Socium. 



This collection consists of some thirty specimens, all of small size, which were 

 mostly taken by means of a net from holes in the ice, while the ' Discovery ' was 

 in \\'inter Quarters. One specimen, however, was captured with the dredge, the 

 depth being given as 100 fethoms. That the Indk of them belong to a single 

 species of Phascolosoma is quite certain, both from their general appearance and 

 from a study of the relations of the various organs, though it is to be noted that 

 these relations have l>een found to be far from invariable. Some half-a-dozen, 

 however, have Itecn the cause of eonsideral)le dithculty in the exact determination of 

 their specific identity ; yet it has seemed best to include them under the aljove 

 species. In the first place, they agree in the main details of their structure with 

 the typical forms; and I have not l)een able to place them without hesitation 

 under any of the other closely allied species (sub-species or varieties) which centre 

 round the Arctic Ph. margaritaceum, namely, Pli. antarcticum, capsifoi'iae, fuscum, 

 ;/eor;/ianuin, or lageiise. In rhe second place, I find in some of them no structural 

 diti'erences from the typical form ; while in others, such ditierences are hardly greater 

 than those found within the limits of the typical form ; while in regard to the most 

 obvious distinction that separates them, namely, difference of general appearance, 

 I have considered that, in so far as colour is concerned, the difference may be due to 

 methods of preservation (in one case at least corrosive sublimate is given as the 

 preservative), and that, in so far as the greater thickness and opacity of the body- 

 wall is concerned, this is quite conceivably due to the greater contraction of the 

 muscles that limit it interiorly. One has, of course, been influenced in forminsr an 

 opinion on this matter l)y a variety of other considerations. A certain small 

 acquaintance with the various types of iSipunculids enables one to estimate, by a 

 method which is l^eyond description, except in so far as it may be called the method 

 of past experience or the results of practice, how far one may venture to overlook 

 those dift'erences of detail which occur as between individual specimens. Again, one 

 is aide to recall that, while in many Sipunculids certain outstanding features appear, 

 which enal)le one with little difficulty to place the individual considered, yet in others 

 these same features are too indefinite or variable to be of much use. And the actual 

 trulh is that, speaking generally, tlie main features which we rely on as affording 

 vol., IV. y 



