Respiration in Invertebrate Animals. 395 



cavity ; an afferent and efferent vessel, canying red blood, dis- 

 posed in parallelism, accompany this axial framework. In the 

 secondary processes con-espondiug to the teeth of the comb, the 

 two vessels affect the inferior margin, to which the vibratile cilia 

 are limited. These cilia are large and vigorous in action. Those 

 of all the gills conspiring, generate a current in the surrounding 

 water which bears in the direction of the mouth. The cilia 

 answer therefore a double purpose : they ceaselessly renew the 

 aerating element in contact with the branchiae and convey food 

 in the direction of the mouth. It is difficult to avoid contrasting 

 the importance of such results with the incomplex simplicity of 

 such means. Tubicolous and sedentary in their habits, the bran- 

 chial appendages in the Serpula subserve thus at once the two 

 highest functions of the organism. In this genus, Serpula, it is 

 the blood-proper exclusively that is subjected to the respiratory 

 process. 



The branchiae in the allied genus Sabella conform in ultimate 

 structure with those of the Serpulans ; they pi-esent, however, 

 a somewhat modified disposition around the head. 



In S. unispira they exhibit an elegant spiral arrangement 

 around a tapering vertical central pillar, ^yhen fully expanded, 

 no object in nature is more beautiful : the elementary parts are 

 comb-like; the straight processes, describing a graceful cork- 

 screw curve around the axial stem, are multiplied by a double 

 row of lesser filiform appendages ; these latter are richly ciliated 

 on the under surface. They carry each an afferent and efferent 

 vessel : on the margin of extension they are strengthened by a 

 dehcate axial scaffolding of flexible cartilage. They are capable of 

 being folded up in a small compass and withdrawn into the tube : 

 they ai*e extraordinarily irritable and contractile. The feet in 

 this genus bear no branchise. In Sabella a sang vert the 

 branchiae rise above the head in gorgeously coloured and circu- 

 larly arranged plumes : they coincide with those of the former 

 in minute structure. 



Sabella vesiculosa exhibits a slight variation of plan as com- 

 pared with those of the former, in the character of the branchial 

 processes. They are supported upon a pedunculated base : this 

 latter is painted with spots of the gayest colours. In Sabella a 

 sang vert the blood displays a deep grass-green colour ; in the 

 other species it is red. How incomprehensible such capricious 

 freaks of nature ! 



The genera Sabella and Serpula, then, may be thus charac- 

 terized as respects the organs and the manner of breathing. In 

 all species the branchiae affect a cephalic situation ; the blood- 

 proper, and not the chylaqueous fluid, is aerated ; in all, the 

 pedal appendages consist only of setae ; in all, the inferior half 



