8 



AUGUST BRINKMANN. 



(REP, OF THE "MICHAEL SARS" NORTH 



Dinonemertes investigatoris Laidlaw 1906. 

 (Plate I. Figs. 1—3). 

 Synonyms : 

 Dinonemertes investigatoris Laidlaw 1906 (14). 

 Dinonemertes investigatoris Brinkmann 1912 (in Murray and Hjort 



(18) p. 577). 



Prior to the return of the "Michael Sars" with the 

 material collected, this species, a giant form among pelagic 

 nemerteans, was only known from the type specimen taken 

 during the cruise of the "Investigator" near the Laccadive 

 Islands, and briefly described by Laidlaw. 



Had not this description been accompanied by a good 

 habitus figure, it would hardly have been possible to 

 recognise the species: by the aid of this however, and 

 with the further assistance of a series of sections kindly 

 placed at my disposal by Mr. Laidlaw, I was able to 

 identify as belonging thereto a couple of specimens from 

 the material of the "Michael Sars", both female, the 

 one fullgrown, the other young. 



The very considerable size of the species will be seen 

 from the following measurements: 



Length. Greatest breadth. Greatest thickness. 



I 203 mm. 56 15 



II 107 „ 23 7 



Figs. 1 and 2 (Plate I) give a good idea of the form, 

 remarkable for its enormous breadth in proportion to the 

 length, the parallel sides, and the marked flattening of 

 the tail, which forms a true caudal fin. The difference 

 in shape of the fore part of the body as between the grown 

 and the younger specimen (Plate I, Fig. 3) is due to the 

 fact that the latter has cast its proboscis, and that the 

 rhyncocoel has been emptied of the rhyncocoelomic fluid, 

 causing a contraction of the head. 



With regard to appearance in a living state, it is 

 stated that the animal is transparent, with the intestine a 

 bright red. As will be seen from the figures, the trans- 

 parency has been lost in these specimens which were 

 preserved in formalin, the intestine being only visible 

 through the thinner caudal region. 



An examination of the anatomical structure having 

 been made, the results may be briefly summed up as follows: 



The muscle layers of the body wall are but very 

 sligthly developed in comparision with the considerable 

 size of the animal; this is especially the case with the 

 circular layers which nowhere exceed 0.05 mm. in thick- 

 ness. The longitudinal musculature can, in the median 

 region dorsally and ventrally, attain a thickness of 0.5 mm.; 

 laterally, both layers are very thin. Between these layers 

 are interposed a few scattered bundles of diagonal mus- 

 culature. 



The mouth opening is situate subterminally; the 

 oesophagus is lacking. The stomach and pyloric tube, 

 which have no exact mutual limit, have, in the larger 



specimen, an aggregate length of 14 mm., which is 

 relatively short. The stomach is however, capacious, and 

 its wall so folded as to permit a high degree of expansion. 



The intestine is narrow, and furnished with about 70 

 pairs of diverticula, these lacking altogether the ramifi- 

 cations so frequently encountered in other pelagic nemer- 

 teans; the only indication of such development — notice- 

 able moreover in the forepart of the body alone - - is a 

 slight pouch-like protrusion on the dorsal side, to some 

 degree overlapping the edge of the rhynchocoel, and an 

 extremely attenuated rudiment of a ventral branch. 



The intestinal caecum is short, and furnished with three 

 pairs of diverticula, the foremost originating terminally. 



The structure of the proboscis resembles in essentials 

 that common to the remaining pelagic nemerteans; its 

 length is about twice that of the body. The stylet basis 

 is strongly curved, and armed with numerous stylets. At 

 least 30 proboscideal nerves are developed. The proboscis 

 pore is situated terminally, leading into a short rhyn- 

 chodaeum. The proboscis sheath, extending out from 

 this, is likewise short, reaching only to within the middle 

 third of the body. The circular musculature of the proboscis 

 sheath is strongly developed and forms a layer in the 

 outer portion of which the longitudinal musculature lies 

 enclosed, not however, as interwoven bundles, but forming 

 a distinct thin layer by itself. This does not altogether 

 apply to the extreme fore-part in the cerebral region, 

 where the arrangement more resembles that found in the 

 Drepanophorus. 



The vascular system exhibits various peculiarities. The 

 mediodorsal vessel forms a small net of anastomosing bran- 

 ches as it passes through the wall of the proboscis sheath; 

 they reunite however, immediately after passing through, 

 and form again a single, undivided vessel. Metameral 

 vascular anastomoses are lacking. The most remarkable 

 feature in the vascular system is however the course of 

 the lateral vessels. These run, in other forms — excepting 

 only such complications as may arise in the nephridial 

 region — straight down through the sides of the animal, 

 following the lateral nerves, whereas in the present species 

 they are intricately intertwined, forming a close mass of 

 loops, partly surrounding the ovaries, and again extending 

 up between or even above the intestinal diverticula. Exa- 

 mination has shown conclusively that no anastomoses are 

 formed between the loops. The importance of this peculiar 

 arrangement doubtless lies in the fact that it provides a 

 means of conveying to the ovaries a sufficient quantity of 

 nutriment during the development of the eggs, which are 

 exceptionally large; this explanation is supported, inter 

 alia, by the circumstance that the loops increase con- 

 siderably both in number and size with the growth of 

 the animal. 



