August 29, 1907J 



NA TURE 



45: 



quence is which pair ■ ! arms is affected by the heclocotylis- 

 ation, whether the first, third, or fourth pair; next in 

 importance comes the nature of the modification; while the 

 question whether the right or left arm is affected is quite 

 insignificant. It will be our business to consider how far 

 the Danish naturalist's position is justified in the light of 

 our present knowledge. 



An inspection of the facts known up to the present 

 time shows, first of all, that where hcctocotylisation 

 is known to take place it affects either the first, third, 

 or fourth pair of arms ; no instance is yet known 

 where the second pair is modified, except in sub- 

 sidiary relation to another pair, or in one or two rather 

 doubtful cases in which all the arms are said to be modified. 

 It appears, furthermore, that hcctocotylisation of the ihiiti 

 pair is confined to the Octopoda, while the first and fourth 

 pairs are affected in the Decapoda, so that, as far as 

 the main divisions of the Dibranchiata are concerned, the 

 position of the hectocotylus is a correct index to them. We 

 may, however, go a step further still, and point out that 

 in every family, with one exception, the position of the 

 hoetocotylised arm is constant within the limits of the 

 family, so that there is a verv' strong prima facie case for 

 the truth of Professor .Steenstrup's dictum. ■ The difficulty 

 arises when we come to consider the family .SepiolidEE and 

 its allies, and endeavour' to form an idea of their relation- 

 ships to each other. 



Steenstrup was so convinced of the truth of his thesii'. 

 th.-ct he divided the Myopsida into two main divisions 

 according to wiiether hectocotylisation affected the first 

 or fourth pair of arms, and placed the four genera Sepia- 

 darium, Sepioloidea, and Idioscpius (notwithstanding their 

 Scpiola-like form) with Spirilla, apart from Scpinla and 

 Rossia, and along with Sepia and Loligo. It becomes 

 necessary now to inquire how far this classification is 

 justified by what we know of the morphology of the 

 forms concerned. 



It will be convenient to deal in the first place with 

 Spirilla, which has always been of great interest on 

 account of the unique structure and position of its shell. 

 It still belongs to the greatest of zoological varieties, only 

 a dozen specimens with the soft parts having been obtained, 

 of which one alone proved to be a male. This was 

 e.\amlned by Sir Richard Owen, who described the hecto- 

 cotylisation as affecting both the ventral arms, which 

 are much enlarged, exceeding the others both in length 

 and thickness : they are quadrangular in section, devoid 

 of suckers, and the right is much larger than the left. 

 The other arms appear to have a round truncated extremity 

 which may be a secondary modification. The relationships 

 of Spirilla have recently been made the subject of inquiry 

 by Prof. Paul Pelseneer, who completed the memoir in the 

 Challenger reports begun by Prof. Huxley, and by 

 Dr. Einar Lonnberg of Stockholm, who dissected a 

 specimen obtained for him from Madeira by the late 

 Captain Eckman. These two investigators arrived at 

 different conclusions regarding its systematic position. 



Pelseneer regards it as an CEgopsid, Lonnberg as a 

 Myopsid, but the anatomical characters on which they 

 are agreed are enough to show that, at anv rate, these 

 two forms cannot be so closely related to each other as 

 to belong to the same sub-family, or even family. 



With regard to the question at issue between 

 them as to the Gigopsid or .Myopsid nature of Spirilla. 

 I think, on the w-hole. that its reseinblance is to the 

 former rather than to the latter ; but I believe that the 

 branch of the ancestral tree which terminates in Spirilla 

 was given off from the main Cephalopod stem before 

 the CEgopsida and Myopsida, as we now know them, had 

 been separately evolved. Paljcontology reveals a possible 

 descent of Spirilla from a Belemnitoid through such an 

 intermediate form as Spiriilirosira ; and from this, on the 

 other hand, it is easy to conceive of the descent of Sepia 

 through a form resembling Bclosepia. Such a relation 

 could be expressed by the following diagram, which is, 

 however, only a rough illustration of possibilities, for 

 Spiriilirostra is a Miocene form and Belosepia an Eocene, 

 so that the former could hardly be the ancestor of the 

 latter. It is only contended that these forms indicate a 

 possible line of descent. 



Spirula 



Sepia 



Belosepia 



I. 

 Spirulirostra 



Belemnites 



Unfortunately, in the present state of our knowledge, 

 it is impossible to correlate the above diagram with one 

 based upon the study of the soft parts of recent forms. It 

 is suflicient if they do not contradict each other. We know 

 nothing of the soft parts of the fossils, and there is no 

 recent form, which exhibits shell characters, bridging over 

 the gulf between Septa and Spirula. To sum up, Spirula 

 must be regarded as, at all events, the representative of a 

 distinct family : it is not unlikely that it mav one day 

 become the type of a division coequal with .Mvopsida and 

 CEgopsida, and it does not appear to me that the structure 

 of its hectocotylised arms would be any argument against 

 such a view. 



We may now consider the genera Idiosepiiis, Sepia- 

 dariuin, and Sepioloidea, regarding w^hich there can be no 

 doubt that on morphological grounds the.se three genera are 

 more nearly allied to the Sepiolidae than to the Sepiidae or 

 Loliginidae ; in fact, practically the only character of any 

 importance which points in the opposite direction is the 

 hectocotylisation. This portion of the subject has been 

 very fully and clearly handled by Dr. Appellof of Bergen, 

 and to his memoir I refer those who desire more de- 

 tailed information. We have here, then, a case in which 

 forms the ventral arms of which are hectocotylised are. 

 more nearly related to forms with dorsal hectocotylisation 

 than to others with ventral, and this shows that the position 

 of the modified arm (or arms) is not by itself an infallible 

 guide to systematic affinity. It is a striking instance of 

 an aphorism of the late Prof. RoUeston, that " no single 

 character can be regarded as a safe basis for a natural 

 classification until it has been proved to be .so." 



It may, however, be worth while to look a little further 

 into the relationships of these forms, and to see whether 

 the hectocotylisation of the dorsal arms is quite as sporadic 

 and irregular as it at first appears. 



After the separation of Idiosepius two possibilities pre- 

 sent themselves as to the further evolution of this group. 



A. The main stem divided into two branches leading to 

 Rossia and Sepiola on the one hand, and to Sepiadarium 

 and Sepioloidea on the other. 



B. The stem gave off first a branch leading to Rossia, 

 and subsequently divided into two, one leading to Sepiola 

 and the other to Sepiadarium and Sepioloidea. 



These two alternatives mav be expressed graphically 

 thus : 



(A) 



Sepiadarium Sepioloidea Sepiola 



I I 



Idiosepius 



Rossia 



I 



NO. T974, VOL. 76] 



(B) 



