6 ARKIV FÖR ZOOLOGI. BAND ?. N:0 30. 



(Vol. 8, p. 270): »Abdomen dilatable into a very large, com- 

 pressed, pendant sac, the lower part of which is merely a 

 flap of skin, into which the air does not penetrate. The sac 

 is kept expanded by the very long pelvic bone.» It is im- 

 possible to judge from this short notice, whether Gunther 

 has dissected any specimens and observed an air-sac or not. 

 Perhaps this form is pro vid ed with a dilated stomach as in 

 Monacanthiis setifer and trossulus, but it is not unhkely that 

 the condition is the same as in M. occidentalis which has no 

 air-sac, but the ventral body-wall behind the pelvic bone is 

 highly dilatable. This movable skin flap is quite solid, the 

 body cavity not entering into it (Fig. 4). It is not certain 

 that Gunther has observed a living specimen, but rather 

 that he has made the remark from the examination of a 

 preserved specimen. It may, perhaps, be pointed out that 

 the 4 specimens of the species in question which are quoted 

 in Gunther's Catalogue, to be kept in the Brit. Nat. Hist. 

 Museum all are stuffed. Dareste does not mention anything 

 about an air-sac. 



Tetrodoii. Thilo has described the air-sac in T, rubripes 

 (Fig. 2 d), which according to him forms the next stage in 

 specialization from that in Monacanthus trossulus. The sto- 

 mach has' turned round, the pylorus thus being placed on the 

 posterior wall of the stomach. It would be of great interest 

 if this account were found to be quite correct, but I am not 

 quite convinced that such is the case. In Spheroides, a genus 

 which is very closely allied to Tetrodon and by some authors 

 regarded only as a subgenus of this, the air-sac is much more 

 developed, being united with the ventral body wall, a condi- 

 tion, that will be described below. A similar connection with 

 the body wall I have found in Tetrodon ocellatus, a species 

 which seems to be very closely allied to T. rubripes. Thilo 

 has, perhaps, not noticed this condition, but only observed 

 that there is a highly enlarged stomach with the pylorus 

 placed on its posterior wall. If in T. rubripes the enlarged 

 stomach or the air-sac, as it may be called, has not united 

 the body wall, this species represents a stage between the 

 specialized 3Ionacanthus-species and Tetrodon ocellatus (and 

 with all probabiHty several other species). We would then get 

 a contjnuous series of different degrees of specialization from 



