163 



the male is attached ; and after mating, she attaches- 

 herself, feeds until gorged, and then drops off to the ground 

 and there produces a great number of eggs. 



The larvae and the nymphs feed on different individual 

 hosts, and therefore three hosts are necessary for the com- 

 pletion of the life-cycle. 



The organisms of ** heartwater " disease are sucked up 

 with the blood of an infected ox by the larva, and are 

 transmitted to the second host by the nymph. If the 

 nymph feeds on an infected animal, the disease organisms 

 may be conveyed, when the nymph becomes adult, to 

 the third host. The organisms are not transmitted through 

 the egg. 



(For modes of infection by Ticks see " Catechism " Zoology > . 

 Part I., pages 47 and 66.) 



PHYLUM ECHINODERMATA. 



What are Echinoderma ? State their General Characters. 



They are prickly-skinned marine animals of sluggish 

 habit, which outwardly exhibit radial symmetry ; they 

 have a limy body integument, with a more or less developed 

 dermic exoskeleton of limy plates or spicules, and the ex- 

 tensive coelome or body-cavity is divided into two distinct 

 portions, namely, the perivisceral cavity (the body-cavity 

 proper) and the characteristic water-vascular system or 

 hydrocoel. Additional parts of the coelome are the axial 

 sinus and the perihaemal spaces, which form an ill-defined 

 system of " blood sinuses." In the coelomic fluid there 

 are numerous wandering amoebocytes. There are no special 

 excretory organs. The sexes are separate. The gonads or 

 generative glands open directly to the exterior ; and the 

 reproductive elements are discharged into the sea, where 

 the eggs are fertilised. There is generally a free-swimming; 

 larva, which outwardly exhibits bilateral symmetry. 



