CUM L'AllATl VE AN AT< >M V. 



161 



COMPARATIVE ANATOMY. XII. 



CEUSTACEA. 



IN the last four lessons we have been tracing upwards the pro- 

 gressive modifications which occur among thoxo animals which 

 have bcou called Articulata on account of thrir joind <1 .'\i. m.,1 

 skeleton, and Homoganyliata from thuir repeated uml Htinilar 

 nervout manses. It will be observed that after leaving the in- 



I. 



prepared and educated for iU final and fully developed stage. 

 It would be almost as false to call these insect* aquatic as to 

 peak of the condition of the higher *nim*U in their state before 

 birth as if it were the final one. In some eases, however, as we 

 have seen, the perfect insect is found in water; but even io 

 these oases the creatures have rather invaded the watery domain 

 than been born to it. Thus the water-beetles rise in the night 

 ami lly from pool to pool, and while underwater must frequently 



1. LOBSTER WITH ONE SIDE or THE CARAPACE REMOVED TO SHOW THE GILL-CHAMBER, LEGS AND LIMBS REPRESENTED AS IF cm SHORT. 



II. LOBSTER REPRESENTED AS IF CUT THROUGH A LITTLE ON THE NEAR SIDE or THE MID-LINE, ONLY CEPHALO-THORAX REPRESENTED. 



III. LEG OF LOBSTER, WITH FLAP AND GILLS. IV. PUAWN (CRANGON) AND ITS SEPARATE PARTS DETACHED. V. CTCLOPS WITH KGG-POUCHM. 

 Eef. to Nos. in Pigs. I. 1, ovary ; 2, liver; 3, gills ; 4, flabellum. II. 1, jaw; 2, throat; 3, stomach; 4, intestine; 5, liver; 6, heart, with slite 



to receive venous blood ; 7, artery to head ; 8, artery to stomach ; 9, artery to near lobe of liver ; 10, artery to upper part of tail ; 11, sternal 

 artery dividing below into 12, sternal abdominal artery ; 13, foot and gill artery. III. 3, 3, gills; 4, flap. IV. 1, eye; 2, 3, antenna;; 

 4 9, foot jaws ; 10 14, walking limbs. 



definitely elongated diffuse and limbless animals known as worms, 

 wo have all along been describing creatures whose life is passed 

 in the air. We have, as it were, seen the articulate typo pro- 

 ceed from its sprawling procumbent condition, gather itself 

 together, put forth limbs, and endue itself with armour, perfect 

 its organs of sense, enter upon the domain of the air, and finally 

 take wing, and vie with the swiftest bird in precision and power 

 of flight. All these animals from the Julus upwards are formed 

 for life in the refined and rare medium of the atmosphere. It is 

 true there are some apparent exceptions to this rule. Thus the 

 larva state of many insects is passed in water. This, how- 

 ever, is an immature state, during which the animal is being 



*63 N.E. 



protrude their tails into the air and breathe through the spira- 

 cles there situated. The water-scorpion (nepa) and boatman 

 (notonecta), though they do not often emerge from the streams in 

 which they live, nevertheless continually resort to the surface 

 for air. The former carries a supply of air in the groove which 

 lies between the down-turned edges of its broad, shield-like 

 upper integument and the abdominal surface of its body, while 

 the latter has the faculty of investing itself with a coating of 

 air in some mysterious way, and carrying it down into the 

 depths below. These semi-aquatic creatures breathe air by the 

 same system of trachea as other insects do, and, moreover, their 

 mode of life is exceptional to the class. Aa a rule, insects 



