428 CHARACTERS OF INVERTEBRATE ANIMALS 



brate (see p. 38), between a blood-system and a lymph-system. 

 The blood- system contains red blood, the colour being due to 

 the presence of the same pigment (haemoglobin) as that found 

 in Vertebrates (see p. 265), though here it is dissolved in the 

 plasma, or liquid part of the blood, and is not contained in cor- 

 puscles. The vessels which carry the blood form a closed system, 

 the smaller branches of which break up into capillary net-works. 

 No heart is present, and the pumping is effected by the larger 

 vessels, along which waves of contraction pass, forcing the blood 

 onwards, much as digesting food is carried onwards in an intes- 

 tine by peristaltic contraction (see p. 37). "The two most impor- 

 tant blood-vessels are longitudinal in direction, one being a dorsal 

 vessel above the gut and the other a ventral vessel below it. The 

 blood flows forwards in the dorsal vessel and backwards in the 

 ventral one. From these two chief trunks transverse vessels are 

 given off regularly in segmental order, and the branches of these 

 break up into net- works in the substance of the various organs. 



The lymph system consists of the body-cavity, and the colour- 

 less lymph which it contains consists of plasma in which float 

 numerous irregular lymph corpuscles (see p. 42). 



Respiration is effected in the body-wall, which is richly pro- 

 vided with blood-vessels branching just below the epidermis. 

 Most likely the foot-stumps play an important part in regard 

 to this function. 



Excretion of nitrogenous waste is performed by the excretory 

 tubes or nephridia, of which a pair are present in almost every 

 segment. Each of these organs is essentially a ciliated tube 

 which begins by a small funnel placed just in front of one of 

 the partitions which cross the body-cavity, runs back to pierce 

 this, and ultimately opens to the exterior on the under surface 

 of the body close to the base of a foot-stump. 



The Central Nervous System consists of a nerve -ring en- 

 circling the commencement of the gut and a ventral nerve-cord, 

 resembling, therefore, the corresponding organs of a lobster. The 

 nerve-ring is thickened dorsally into a closely-fused pair of brain- 

 or cerebral-ganglia, and the ventral cord, which is of double nature, 

 swells into a pair of closely approximated ganglia in each segment. 



Sense Organs. The cirri and head-tentacles are presumably 

 organs of touch. It is likely that the palps borne on the head 

 have to do either with taste or smell, or it may be with both. 



