LIFE HISTORY STUDIES OF ANIMALS. 485 



nervous system lies on the outside of the body, and that in other ani- 

 mals it is generally to be found there during some stage of development; 

 that there are animals whose circulation reverses its direction at fre- 

 quent intervals either throughout life (tunicata) or at a particular 

 crisis (insects at the time of pupation) ; that there are animals with eyes 

 on the back {Oncidium, scorpion), on the shell (some chitonidse), on 

 limbs or limb-like appendages, in the brain cavity, or on the edge of a 

 protective fold of skin ; that there are not only eyes of many kinds with 

 lenses, but eyes on the x)rinc:pleof the pin-hole camera without lens at 

 all {Nautilus) and of every lower grade down to mere pigment spots; 

 that auditory organs may be borne upon the legs (insects) or the tail 

 {Mysis) ; that they may be deeply sunk in the body, and yet have no 

 inlet for the vibrations of the sonorous medium (many aquatic ani- 

 mals). It is well that he should know of animals with two tails 

 {Cercaria of gasterostomum) or with two bodies permanently united 

 (Diplozoon) ; of animals developed within a larva which lives for a 

 considerable time after the adult has detached itself (some starfishes 

 and nemertines) ; of animals which lay two {Bajjhnia) or three kinds 

 of eggs (rotifera); of eggs which regularly produce two {Lumbricus 

 trapezoides) or even eight embryos apiece (Praojnis^); of males which 

 live parasitically upon the female (cirripedes), or even undergo their 

 transformations, as many as eighteen at a time, in her gullet {Bonellia) ; 

 of male animals which are mere bags of sperm cells (some rotifera, 

 aome Ixodes, parasitic copepods), and of female animals which are mere 

 bags of eggs [Sacculina, Entoconclia). The more the naturalist knows 

 of such strange deviations from the familiar course of things the better 

 will he be prepared to reason about what he sees, and the safer will he 

 be against the perversions of hasty conjecture. 



If a wide knowledge of animals is a gain to physiology and every 

 other branch of biology, what opportunities are lost by our ignorance 

 of the early stages of so many animals! They are often as unlike to 

 the adult in structure and function as if they belonged to different 

 genera, or even to different families. Zoologists have made the wildest 

 mistakes in classifying larvae whose subsequent history was at the time 

 unknown. The naturalist who devotes himself to life histories shares 

 the advantage of the naturalist who explores a new continent. A 

 wealth of new forms is opened out before him. Though Swammerdam, 

 Reaumur, De Geer, Yaughan Thompson, Johannes Miiller, and a crowd 

 of less famous naturalists have gone before us, so much remains to be 

 done that no zealous inquirer can fail to discover i^lenty of untouched 

 subjects in any wood, thicket, brook, or sea. 



Whoever may attempt this kind of work will find many diificulties 

 and many aids. He will, of course, find abundant exercise for all the 

 anatomy and physiology that he can command. He will need the 



1 Hermann von Jhering, Sitz. Berl. Akad., 1885; Biol. Centralbl., Bd. VI, p. 532- 

 539 (1886). 



