16 



NATURAL HISTORY OF ARTHROPODS. 



two genera, Ibla and ScalpeUum are generally solitary ; but in these genera we have 

 a queer assortment of sexes. First, we have the normal hermaphrodites ; secondly, 

 forms similar in appearance but which are only females, the testicular tissue and the 

 intromittent organ being absent ; third, males which are attached to the females, and 

 others (complemental males) which are found only in the hermaphrodites. These 

 males show a great imperfection of development, and always are found living like 

 parasites. In some the mouth and alimentary canal is entirely lacking, some have a 



peduncle, others none. The male generative 

 organs are always developed, but the intro- 

 mittent organ when present is very short. 

 These forms are found just within the valves, 

 there being sometimes ten complemental males 

 found within the capitulum of a single her- 

 maphrodite. Upon the theory that cross-fer- 

 tilization is a necessity, the existence of these 

 different forms is readily understood; upon 

 every other it seems to be inexplicable. Nor 

 is our knowledge of cross-fertilization wholly 

 theoretical. Both Dr. Fritz Mtlller and Mr. R. 

 Bishop have published accounts of witnessing 

 the very act, and the writer would also add 

 his testimony, having seen the operation in the 

 common white acorn barnacle, £cdanus bala- 

 noides. 



We have noticed the curious life-histories 



which the older writers connected with the 



Fig. 20.— Naupiius of jBaianus 6a!o7ioi(ies, the Barnacles, but scai'cely less marvellous is 



frontal horns not fully extended ; greatly enlarged, ^j^^ ^^^^ embryology of the grOUp. The fer- 

 tilized eggs, which we left inside the capitulum, there undergo an unequal segmenta- 

 tion, some of the cells growing around others which are destined to form the 

 alimentary canal. Soon three pairs of legs bud out, the two posterior becoming 

 bu'amous, and the young barnacle hatches in the form known as a NaupUus. This 

 nauplius is characterized by the presence of two frontal horns, a single eye on the 

 under surface of the head, and a long upper lip. All cirriped nauplii have these points 

 in common, but otherwise they present many prominent points of difference in the 



different groups. In some- there is a great develop- 

 ment of spines. With its growth the nauplius moults 

 several times, and finally it passes by a single moult 

 into the Cypris stage (so called from its resemblance 

 to the adult condition of one of the Ostracoda). The 

 dorsal shield is then replaced by a bivalve shell, 

 hinged above and kept closed by a transverse muscle 

 just below the mouth; a compound eye appears on 

 either side of the primitive single ocellus. A fourth pair of appendages arise behind the 

 mandibles, and behind these are found the rudiments of six pairs of feet. The second pair 

 of antennae disappear, while on the first pair is developed a sucking disc, in the centre 

 of which is the opening of the cement gland. With succeeding moults the posterior feet 

 increase in. size, and are used in swimming. The free-swimming cypris stage is not of 



Fig. 21. — Cypris stage of Balanus, 

 enlarged. 



