376 Messrs. Alder and Hancock on the Branchial 



distribution of the channels within the laminae. The free mar- 

 gins, down upon which we looked, were rather thick and di- 

 stinctly longitudinally grooved. Along this marginal groove a 

 minute cord or stream of blue particles of indigo was observed 

 moving towards the mouth, following accurately all the sinuo- 

 sities of the much-undulated gills. Minute streams of similar 

 particles were also observed coming up from the base on both 

 sides of the gills to join the stream that continued its progress 

 along the mai-gin. These lateral streams passed for the most 

 part up the furrows of the lamina?, and becoming incorporated 

 with that on the margin, augmented its volume as it approached 

 the anterior extremity of the gill. All these streams were sharply 

 defined, and the particles of which they were composed did not 

 move amongst themselves, but were formed into cords or threads 

 rather than streams, apparently bound together by some tena- 

 cious fluid. These streams or cords continued to be formed and 

 moved on for hours ; the action being only occasionally retarded 

 when the animal was disturbed, and the whole time the water was 

 continually passing out of the excurrent orifice or anal siphon. 



Thus considerable quantities of indigo were accumulated in 

 the vicinity of the mouth and oral tentacles. These accumula- 

 tions were composed of ravelled threads spun as it were by the 

 branchial apparatus, from the scattered, nearly invisible particles 

 of indigo in the surrounding medium. 



On reviewing this curious spectacle the question naturally 

 arises — how is the matter, divided into such minute particles, 

 thus collected on the surface of the gills ? Having arrived at a 

 clear understanding of the structure of these organs, we can give 

 a correct answer to this question. 



We have seen that the two laminse forming the walls of the gill 

 are in fact in structure like sieves, and that the tubes which pass 

 between them communicate with the anal chamber leading di- 

 rectly to the excurrent siphon. When the water is flowing out 

 of this siphon, there is consequently a tendency to form a vacuum 

 in the chamber and in all the tubes between the gill-laminse. By 

 this means, combined with the action of the cilia lining the meshes, 

 the water introduced into the branchial chamber by the inhalant 

 tube will be drawn through the meshes of the laminjBj and all 

 the suspended particles of matter retained on the surface of the 

 gill. The water being filtered, as it were, allows the particles to 

 accumulate, and then by the aid of other cilia, they are formed 

 into the defined streams above described. This is certainly one 

 of the most beautifully adapted organic mechanisms that can be 

 looked upon. As a breathing organ it is very complete ; as a 

 prehensile organ for securing food it is surely unrivalled for the 

 minuteness and beauty of its structure. 



