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. IV. On the external Anatomy of Tanais vittatus, occurring tvith Limnoria and Chelura 

 terebrans in excavated Pier-ioood. By John Denis Macdonald, M.D., F.B.S., 

 Staff-Surgeon B.N., Assistant Professor of Naval Hygiene, Netley Medical School. 

 (Communicated by W. T. Thiselton Dyer, M.A, F.L.S.) 



(Plate XV.) 

 Eead Feb. 18th, 1875. 



VV HILE searching for- Limnoria terebrans in excavated wood from Netley Hospital 

 Pier my attention was diverted to another little Crustacean, rather smaller in size, but 

 wearing the outward appearance of a chelate Decapod so completely that it was only by 

 counting its legs and more minute inspection of the leading features of its structure I 

 was enabled to determine it to be a veritable Isopod, though of an aberrant type. 



It proved to be conformable with the characters of Tanais Cavolini (E dw. )=Crossurus 

 vittatus (Bathke), which was originally found in the Gulf of Naples, the habitat assigned 

 to it by Milne-Edwards ; but it has since been shown to have a wider range. Its occur- 

 rence under the above-mentioned circumstances is worth recording ; for I have also found 

 it in perforated and decayed wood from the pier at Alum Bay, Isle of "Wight ; and I have 

 no doubt that it would be captured in other localities if it were sought for in the burrows 

 of Limnoria. 



Quite exceptionally, in the Tanaidae the head and first thoracic segment coalesce above 

 to form a perfect little carapace or cephalothorax, which, in the little creature under 

 consideration, is somewhat triangular in form, with an obtuse but distinct rostrum in 

 front, from beneath which the two pairs of antennae project forwards ; and immediately 

 to the right and left of these are the orbital notches for the eyes. 



These latter organs are very prominent, in articulation with the carapace, and all but 

 pedunculated. They are conical in form, with the apex directed forwards, and rather 

 more convex or rounded on the outer than on the inner side. Round lenticular corneas 

 not unlike those of Limnoria, but much smaller, are recognized by their brilliancy, and 

 the relief of a central mass of black pigment radiating amongst the little nervous cups in 

 connexion with them. 



Having thus commenced with the eyes, I shall describe the members of the succeeding 

 segments seriatim, which will render their homological relations more intelligible. 



The two pairs of antennae (figs. 1, 2, & 3) are nearly of equal length, rather shorter 

 than the cephalothorax, and disposed in two lines, one above another. The superior 

 antennae (a, figs. 2 & 3), which are much stouter and somewhat longer than the inferior, 

 are each composed of three joints, successively diminishing in their proportions, and 

 setaceous at their distal ends. Several bristles of a ligulate form (fig. 2, e), with transverse 

 striae near their base, occur amongst the simple ones in the tuft at the tip of the terminal 

 segment. A labyrinth-like excavation at the upper and inner side of the base of the first 



SECOND SERIES. — ZOOLOGY, VOL. I. L 



