2 W. F. LANCHESTER. 



diiferentiating specific characters in this group are ultimately of a very vague and 

 unsatisfactory kind ; while they appear clear enough in many cases, yet in 

 others we at once realise how indefinite they are. We are dealing with very contractile 

 animals, and yet use such points as the relative lengths of the body, and the 

 introvert ; nay, the very limits between these two are generally indefinable, and even 

 were they not, still, one portion may be relatively more contracted than the other. 

 The length of the segmental organs relative to other structures has its value, yet 

 I cannot help feeling that a certain amount of extensibility must be conceded to them ; 

 and the question of their colour surely affords an absolutely valueless criterion. 

 So with the number of coils of the gut ; extremes of number indicate a 

 difference undoubtedly, but one is naturally suspicious of such distinctions as 

 lie between 16, 18, 20, and so on. Thus the present specimens (out of those 

 that are undoubtedly similar) give us a length of segmental organs varying from 

 1*5-4 mm., and number of gut-coils varying from 12-25. So that when Dr. 

 Michaelsen tells us that two of the differences between Ph. aniarcticum and Ph. 

 fuscum are that the latter has " etwa 18" and the former " ungefahr 20 Doppel- 

 windungen," and that in the latter the segmental organs are " weit langer als die von 

 Ph. antarcticum," we feel that the first distinction is valueless, and that the value of 

 the second must depend on the amount of the other distinctions, which are as a 

 matter of fact admitted to be slight as between the two species. I trust I may not 

 be taken to mean that the characters usually quoted as of specific worth have no value. 

 My object is, in the main, to indicate the considerations that have induced me to join 

 up in a particular instance what others might have been inclined to separate ; though 

 it is naturally obvious that I tend to consider that certain details that are universally 

 quoted in specific descriptions of Sipunculids should be subjected to very careful 

 consideration before they are accepted as having any absolute value. The absence of 

 longitudinal muscles, and, in most species, of hooks within the genus Phascolosoma is 

 doubtless one of the reasons why it is more unsatisfactory to deal with than the other 

 genera of the family. 



The present species then presents in the main the chief features of the Ph. 

 margaritaceum group. From Ph. margaritaceum and Ph. capsiforme it seems to be 

 distinguished by having the introvert not much shorter than, instead of being only 

 half, or a little more than half, the length of the body ; yet here I would like to point 

 out that I have examined three specimens of Ph. capsiforme which were kindly sent 

 to me, through Prof. F. J. Bell, by Dr. Michaelsen. Now in one of these, in which the 

 introvert is fully extended, it appears from careful measurements that the introvert is 

 " approximately " equal in length to the body. I say " approximately " only, because 

 of two difficulties : (a) that the introvert is twisted round on itself ; and (b) that, 

 simple as it seems to distinguish the confines of introvert and body when regarding 

 the object with the ordinary eye, yet the application of a measuring instrument at once 

 reveals the difiiculty of deciding on a real line of demarcation between the two. 



