BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 339 



Dr Johnston* observes of this species, that " the structure of the stomach " 

 (proboscis) " is like that of its congeners, excepting in there being five or six 

 spines on each side of it, instead of three, which is the usual number." He does 

 not refer at all to the remarkable arrangement of the central stylets, though an 

 incomplete woodcut in one of his early papersf shows that it had not entirely 

 escaped the notice of his accomplished artist. 



The general arrangement of the proboscis in Tetrastemma algm agrees with 

 that in 0. alba, though there are some minor differences in the details of the 

 stylet-region. If under examination the ejaculatory duct is placed on the left of 

 the central stylet-apparatus, an explanation is obtained of the mistake into which 

 M. Claparede^: had fallen in his description of the region in Tetrastemma vari- 

 color, (Erst, (the figure, however, appears to me to be very like that of T. alga?). 

 The central stylet and its sac have been slightly pressed backwards so that the 

 radiating fibres which sling them have been brought out distinctly, and some- 

 times a faint line of demarcation is seen on the right side (in such a position) 

 simulating the presence of a separation ; but numerous fibres are prolonged past 

 this, and, moreover, a slight contraction or change of position obliterates this 

 line, while the curved or radiating fibres are rendered more distinct. On the left 

 side the only boundary line to the supposed distinct coat around the wedge- 

 shaped setting is the wall of the ejaculatory duct. The basal sac of the central 

 stylet in T. algm (to continue the description) has rather more shape than in 

 0. melanocephala, and is proportionally more elongated. I thought I could detect 

 a slight difference between this species and T. variegatum, for the stylet in T. 

 algm is generally shorter in proportion to the length of the sac than in T. varie- 

 gatum. Considerable variations exist in the size of the several stylets in T. 

 algm, independently of the size of the animal, a fact, perhaps, the less to be 

 wondered at when the reproduction of the tube is remembered ; but the greater 

 size is generally diagnostic when compared with other species. In a developing 

 or recently repaired central apparatus (Plate V. fig. 14) the basal sac is thinned 

 off anteriorly from contraction of the parietes, and the difference in size between 

 this central stylet and one from the lateral stylet-sac (Plate V. fig. 15) of the same 

 animal is marked. In Tetrastemma variegatum the structure of the stylet-region, 

 while agreeing generally with 0. alba, is yet more particularly allied to T. algm. 

 The stylets are on the whole more slender than in the latter, and the central longer 

 in proportion to its basal sac. In T. vermiculus the structure is similar to the 

 two former (Plate IX. fig. 12). The shape of the basal sac of the central apparatus 

 in T. varicolor is characteristic (Plate VI. fig. 5), the stylet being more slender 

 than in the other two species, larger in proportion to the sac, and the lateral lines 

 of the latter nearly straight. The proportionally large size of the glands in the 



* Catalogue of Worms, &c. p. 292. f Mag. Zool. and Bot. vol. i. p. 531, fig. 4. 



% Recherches Anat. sur les Annel. Turb., &c. p. 81, plate v. fig. 6. 



