34 Dr. T. Williams on the Mechanism of Aquatic 



IV. — On the Mechanism of Aquatic Respiration and on the 

 Structure of the Organs of Breathing in Invertebrate Animals. 

 By Thomas Williams, M.D. Lond., Licentiate of the Royal 

 College of Physicians, formerly Demonstrator on Structural 

 Anatomy at Guy^s Hospital, and now of Swansea. 



[With two Plates.] 



[Continued from vol. xiii. p. 312.] 



Mollusca, 

 The Annulose and Arthropodal series conduct by a separate 

 path across that wide space which divides the Echinodermal from 

 the Vertebrate animal. The MoUuscan subkingdom traverses 

 the same distance by a divergent route which begins at the 

 Bryozoon and terminates at the Cephalopod. These grand inver- 

 tebrate chains of beings unite mutually below at the Echinoderm 

 and Bryozoon, and superiorly at the basilar link of the verte- 

 brated series. The zootomist, having studied serially the arti- 

 culate families^ is constrained to return to the base of the inver- 

 tebrate cone, in order to seize the point of departure of that 

 independent road along which the moUuscan families attain the 

 summit. Between these groups there exist few points of inter- 

 communication. Reciprocal affinities nowhere attract attention. 

 The Mollusca constitute a separate study : in varieties of form 

 they are equalled by no other division of invertebrate animals ; 

 in number of species they exceed almost the limit of arithmetic ; 

 in diversities of structure they bewilder the anatomist ; in 

 modes of life discordantly diverse, they perplex the student of 

 their habits. And yet a deeper insight into the plan of the 

 moUuscan organism enables the earnest thinker to seize the clue 

 of natural union which obtains between the countless meipQbe^;s 

 of this variegated group. forrf? 



Provided with a heart to circulate the blood, a distinct aliment- 

 ary system, a nervous system, and its satellitic organs of sense, a 

 muscle-apparatus, viscera of complex organization, and a blood- 

 fluid fibrinized and corpusculated, they offer to the physiologist 

 a problem by no means easy of solution. 



Of this composite machinery the respiratory function is the 

 primary moving power. Without it nothing can go on. It is 

 momentarily important. What provisions are made to insure 

 its full and adequate performance ? The terrestrial Gasteropods 

 excluded, all mollusks respire on the aquatic principle. They are 

 tenants of the water. The organs of breathing in bulk and com- 

 plexity of structure far surpass those dedicated to other offices. 

 The bulk of blood, which at any given time is included within 



