On the Structure of the Mouth in Sucking Crustacea. 295 



XXV. — On the Structure of the Mouth in Sucking Crustacea. 

 By Prof. J. C. ScHioDTE. 



[Continued from p. 266.] 



20. The third type is that of Hyperini—o. modification of 

 the general type of Amphipoda, adapted for life in the light 

 (large eyes) and powerful swimming about at the surface of 

 the ocean. A parallel to this could not be expected amongst 

 the heavy Isopoda ; but we find one amongst Ulonata, where 

 the Odonata occupy an exactly analogous position to that of 

 Hyperini amongst Amphipoda. Their well-known teeming 

 variety in general external appearance, from the thick-set 

 form resembling a bean, to the most slender and elongate 

 shapes, as well as in the development of the limbs for prehen- 

 sion, climbing, and attachment, is explained by the great 

 variety of structure and mode of life of those (mostly gela- 

 tinous) marine animals to which they attach themselves. 

 Their true relations to these are probably not yet fully eluci- 

 dated; but the following account of the structure of their 

 mouth will show that at any rate they appear extremely 

 well equipped for jDeeling off and gulping down little bits of 

 the bodies of such animals. 



In illustration of this type we may examine the head of 

 Themisto libellula, Mandh Viewing it straight in front, we 

 observe at once the analogy with the head of Odonata. The 

 front, properly speaking, carrying the two pairs of antenna, 

 is deeply sunk between the eyes ; and below it the clypeus is 

 seen to protrude like a hood ; the terminal portions of all the 

 appendages of the mouth are, as it were, folded together so as 

 to form a perpendicularly descending inverted cone; the stipites 

 of the mandibles form a slightly trisinuate frame on either side 

 of the flat bilobate upper lip ; whilst the mandibular palpi, 

 when at rest, fit closely under the lateral margins of the 

 clypeus, the slender middle joint of each ascending perpendi- 

 cularly in the hollow of the front, and the small pointed 

 terminal joint crossing its opposite neighbour below the upper 

 antenme. Below the upper lip the palpi and the apices of the 

 stipites of the first pair of maxilla are seen somewhat fore- 

 shortened, whilst the second pair of maxillae are hidden behind 

 the palpi of the first pair and the anterior ends of the lobes of 

 the maxillipeds, which are turned upwards and forwards, con- 

 stituting the downward- pointing apex of the cone formed by 

 the oral limbs. This view already discloses that the lobes of 

 the mandibles are entirely covered by the upper lip. If we 

 next examine the head from the side, we observe moreover 



20* 



