274 SUCTORIAL CRUSTACEA. 



812. We next pass to the Sub-Class of Suctorial Crustacea, 

 in which the mouth, instead of being furnished with mandibles 

 and maxillae adapted for the division of solid aliment, is prolonged 

 into a beak, and can only receive liquid substances. Still among 

 the Crustacea, as in Insects, the parts which compose the Mandi- 

 bulate mouth of the preceding Orders, may be recognised in the 

 Haustellate or Suctorial mouth of those we have now to consider. 

 The proboscis is formed by a prolongation of the upper and lower 

 lip ; and it is generally accompanied by a pair of long appen- 

 dages, which evidently correspond with the mandibles of the 

 preceding Orders, and which serve to make the punctures into 

 which the proboscis is inserted. Other appendages, found at its 

 base, are the representatives of the maxilla and feet-jaws of the 

 Mandibulate Orders ; and these are frequently developed as 

 hooks, by which the animal attaches itself to its prey. Many 

 of the Suctorial Crustacea bear a strong resemblance in their 

 general form to the Cyclops and other Copepoda. The body 

 is usually divided, at its anterior part at least, into distinct 

 segments ; and there are generally four pairs of legs, each fur- 

 nished at its extremity with two oar-like expansions. The 

 resemblance is often strongest in the early stages of development ; 

 the young Lerncea, for example (Fig. 502), being scarcely 

 distinguishable from the young Cyclops (Fig. 496). But in their 

 advance towards maturity, the Suctorial Crustacea frequently 

 undergo the most extraordinary changes ; so that, if we were to 

 attend to their adult forms alone, we should be tempted to 

 exclude them from the class altogether, in so small a degree 

 do they present its distinguishing characters. This group is 

 divisible into three Orders ; of which the first two are parasitic, 

 and the last free. 



X. SIPHONOSTOMA. In this Order, the mouth is armed with 

 styliform mandibles ; the thorax is composed of several distinct 

 joints, and bears three or four pairs of swimming-feet ; and the 

 feet-jaws are well developed. 



XI. LERKLEIDA. The mouth in this Order, as in the preced- 

 ing, is armed with piercing mandibles ; but the thorax exhibits 

 no division into segments ; and both the feet-jaws and the true 

 legs are undeveloped. 



