^61. 



THE ACALEPHAE. 



65 



CHAPTER V. 



DIGESTIVE APPARATUS. 



§61. 



The digestive apparatus of the Acalephae is formed after several very 

 different types. The mouth is sometimes single and central, or there may 

 be many of them. It is often surrounded with arms and retractile filaments, 

 which are endowed with the prehensile and nettling organs just described. 



The digestive cavity, which is always lined with ciliated epithelium, has 

 distinct walls, which are united immediately to the parenchyma of the 

 body, leaving, therefore, no surrounding cavity. 



With 'those having a single mouth the stomach is of a variable size, and 

 has often caecal appendages. With Beroe,^^^ the mouth is very large and 

 free from tentacles, and opens into a very spacious stomach which occu- 

 pies nearly the whole body. But with Cestum, Cydippe and Lesueuria, 

 the stomach is small, and appears like a cavity in the body;'^' and with 

 Cytaeis, Thauma?itias and Geryonia, it is likewise small, and has the 

 shape of a tubular projection.^^' 



That of Medusa has four saccular folds, ^*> that of Pelagia^^^ six, and that 

 of Cyanea thirty-two.*''' 



When the mouths are numerous, either, as in the Rhizostomidse,*'' there 

 are many canals which conduct the food through the arms upon which the 

 mouths are situated into the central stomach ; or, as in the Siphonophora, 

 each mouth opens into a particular tubular stomach. With these last, 

 however, a certain number of their tentacles are hollow, and have a mouth 

 at the extremity. As it has been observed that these suck in food and 

 digest it, their orifices have been regarded as mouths, and their cavities as 

 stomachs. '**' 



1 Milne Edwards, Ann. d. Sc. Nat. XVI. pp. 5, 6. 



2 Esclisckoltz, loc. cit. Taf. I. II. ; and Milne 

 Edwards, loc. cit. fl. III. 



3 mil, loc. cit. Taf. II. 



i liaer, in Meckel^s deutschs. Arch. VIII. 1823, 

 Taf IV. fig. 2 ; also, Ehrenberg in Abhandl. d. 

 Beri. Akad. 1835, Taf. lU. fig. 1. 



5 Wagner, Icon. zoot. Tab. XXXIII. fig. 5. 



fi Gaede, loc. cit. Taf. II. 



7 Eysenhardt, Nov. Act. physico-med. X. part 

 II. p. ayi, Tab. XXXIV. fig. 1 {Rhizostomum 

 Cuvieri). 



8 This is so, for examples, in Diphyes (JVill, loc. 

 cit. Taf. II. fig. 22) ; in Physalia {Olfers AbhantU. 

 d. Berl. Alvad. 1831, p. 162, Taf. I.) ; in Stephana- 

 mia {Milne Edwards, Ann. d. Sc. Nat. XVI. PI. 

 VII. IX. X.) ; and in Pkysophora (Philippi, 

 MiUler's Arch. lS-i3, Taf. V. fig. 1, 4). 



Philippi, however, affirms that in this last 

 genus these canals are organs of absorption, and that 

 the true stomach, which has a simple mouth, is 

 concealed at the base of the tentacles (loc. cit. p. 

 63, Taf. V. fig. 10). 



I think, however, that this opening belongs to the 

 respiratory system, as also does a similar opening 

 in l^elella and Porpita, which Lesson (Voyage de 

 Duperrey, loc. cit. p. 49, 56, No. 6, fig. B. ; and No. 

 7, fig. C. C.) has regarded as a mouth. 



The tubular tentacles of these animals are noth- 

 ing but stomachs ; and Lesson himself has called 

 them "poches stomacales," smce they digest 

 food. It would, moreover, be strange that these 

 organs, whicli, in Physalia, have been admitted to 

 be stomachs, should perform another function in 

 Physophora, f^elella, and Porpita, where their 

 structure is the same. But further researches are 



(Ijoc. cit. p. 316.) On a preceding page he says : 

 " That this may be the case seems probable when 

 we consider the relation of the two sorts of appa- 

 ratus in the two types. The upper nerv»us ring in 

 Sarsia bears the same relation to the central ali- 

 mentary cavity, and to the pigmented disc, that the 

 ganglion and eye-speck of Beroe bear to the chy- 



6^ 



miferous system, which opens above its gelatinous 

 disc, notwithstanding these openings." (p. 248.) 

 Tliis point, fully as interesting from its zoological 

 importance as from its morphological relations, 

 can }x settled only by a knowledge of the embryol- 

 ogy of these animals. — Ed. 



