EEPOET ON THE NET DIBEAN CHI ATA. 
141 
at its extremity by a minute circular orifice (fig. 17) ; the evagination was the anterior 
end of that portion of the tube provided with hooks. This liook-bearing part of the 
seminal duct was '2 mm. in length, its diameter in front being *3 mm., and further back '16 
mm. ; the hooks were arranged as usual in longitudinal quincuncial series (PI. VII. 
fig. 4) ; the number in the rows was tw x enty to twenty-five hooks (twenty in the anterior 
row, twenty-two to twenty-five in the posterior). The colour of these hooks was a 
faint yellow, they were usually strongly bent ; in the anterior part their length was '04 mm. 
The papillae upon wdiich these hooks are formed are conical, broader at the base, and 
running into a point above, upright or bent. They are made up of small nucleated cells. I 
observed a very similar structure in the hooks of Triboniophorus ; 1 Semper 2 also in 
Onchidium. But I do not understand why Semper speaks of this structure as “ cartilagin- 
ous,” and of the hooks as “cartilaginous teeth.” There is no trace here of any real cartilagin- 
ous structure. The penis, as already mentioned, opens laterally into the large sac of the 
dart-gland. The dart-gland (PI. VI. fig. 12, h) is long and thick- walled, and with its many 
coils covers a large portion of the ampulla developed at its end (fig. 12, ii). The whole gland 
can be easily unravelled, and then attains a length of 45 cm. ; its diameter is about 
'75 mm., the windings of the gland are connected by connective tissue. This connective 
tissue appears to start from a low irregular frill, which winds itself spirally round the 
ampulla and fuses with it. The gland has a roundish, rather wide lumen, lined with a 
thick epithelium, and its wall has nearly the same macroscopic and microscopic structure as 
the ampulla. The gland suddenly becomes wider at the ampidla ; this last is sausage- 
shaped, somewhat arched, and at each end rather more slender, dirty yellow coloured ; 
when extended it measures 2 '8 cm. by 5 '5 mm. in diameter. In transverse section the 
lumen of the ampulla appears triradiute (PI. V. fig. 25), lined by the above-mentioned 
epithelium. Sections coloured with picric acid showed the parts nearest the lumen and to 
the periphery most coloured ; the crenate triradiate lumen is embraced by a thick circular 
layer, which was interwoven and surrounded outside by a more or less continuous 
longitudinal layer of fibres ; between the central and peripheral coloured layers are delicate 
rings and arches of tissue, composed of circular fibres imbedded in connective tissue ; 
here and there, especially in the peripheral layers, were spaces for blood-vessels. In 
similarly prepared sections of the gland itself, the structure was precisely similar, only 
the thin middle layer was more strongly developed, and the lumina of vessels more 
abundant. The duct of the dart-gland, w T hich takes its origin from the anterior -part of 
the ampulla (PI. VI. fig. 12, k), is about as thick as the gland, or a trifle thicker, and is 
half as long again as the ampulla, which it resembles in structure ; in front it opens 
(fig. 12, l, 14, ob) into a sac-like somewhat flattened organ about 6 mm. long. At the 
1 R. Bergh, Anat. Untersuch. d. Triboniophorus schutteii, K., Verhandl. d. k. k. zool.-bot. Gesellsch. Wien, Bd. xx., 
1870, p. 853, Taf. xiii., figs. 7-9. 
2 Semper, loc. cit., Landmollusken, Heft v., 1880, p. 253, &c., Taf. xxii. figs. 4, 12, 16, Taf. xxiii. figs. 3, 5, 6, &c. 
